Recommended travel accessories

Highly recommended:

• Mobile Router (wifi hotspot)
  There are plenty of options for getting the internet on holidays. You can buy some relatively reasonable roaming plans with your existing carrier. You can rent a mobile router in your destination. You and your travel companions can also get separate sim cards for all of your smartphones. Or (if you are feeling lucky) you can just rely on local WiFi (and spend most of your holidays searching for the closest McDonald's).
  However bringing your own mobile router and buying local sim card is by far the cheapest and the most convenient at the same time.
  Link: https://www.ebay.de/p/16010969070?iid=273406703771
  (pretty much any 20€ router will do just fine for Google Maps and Tripadvisor)

• Wall plug adapters
  I strongly recommend the universal ones. They are not much more expensive than single-country versions, and you can be sure you've got the correct one, no matter where you go. Just don't wait until the last minute before buying them. If you get them at your local electronics store, they will charge you around 7€. If you buy them at your destination, sometimes you'll have to pay as much as 15€ (especially if you don't know where to look). But if you buy them from China, they will cost around 1.50€.
  Link: https://www.ebay.de/itm/292673798421
  (this one is very good compromise between price and reliability, but there are many other models to suit your preferences)

• Power bankS
  Is it better to get two, but with smaller capacity? Or one, but with greater capacity? Get two, both with as big capacity as possible. Use one of them normally, and have the second one as a backup. 2 times 10000 mAh is a bare minimum. If your laptop supports charging via USB type C or thunderbolt, get a third power bank, and carry it with your laptop.
  Link: https://www.amazon.de/Powerbank-ALLPOWERS-22000mAh-Ladeger%C3%A4t-Smartphone/dp/B07C4RZHG2/ref=sr_1_7?ie=UTF8&qid=1534538126&sr=8-7&keywords=20000+mah
  (there are many cheap power banks with moderately positive feedback. However this one for 15€ is much better deal than what you can currently find on eBay. Especially considering shipping from the EU)

• Multi-port USB charger
  Pretty much removes the need of using power bars with many plugs. I would recommend getting a charger with at least 4 ports. They are not much bigger from usual single-port chargers, and will allow you to charge all your devices from just one power socket.
  https://www.ebay.de/itm/262616457194
  (offers with EU shipping seem to be very overpriced, and start from ~10€. But you can check your local offers, perhaps you will find something more reasonable)

• UBS hub
  It's a good replacement for separate multi-port USB charger, when the power source is far away from you. For example, you are at McDonald's, and you connected your laptop to the power plug 2 meters from you. Charging via USB dongle from your laptop makes it possible to use your phones and other devices at the table. There's a good chance you will lose this dongle at some point, so don't waste money on features that are useless during trips (such as ethernet port, headphone jack, bluetooth module, etc.). As many USBs as possible, maybe one thunderbolt, and you're good to go.
  https://www.ebay.de/itm/183348137068
  (cheap 4-USB hub, no quick charge, no thunderbolt, the simplest of the simplest)

Worth considering:

• Portable travel scale
  You will probably use it once, at the end of your trip to check the weight of your luggage. But it will save you a lot of worries the night before your flight back home. And also spare you the trouble of repacking on the airport in case the weight was greater than you expected.
  https://www.ebay.de/itm/172872280832
  (this one caught my attention, as it is powered by readily available AAA batteries, rather than button cell batteries)

• Kindle
  Great battery life, fantastic reading experience, reasonable price. On Kindle Fire can also watch movies and play some games. Of course, you can do all of that on your smartphone. But if you want to feel safe about your remaining battery life and have dedicated device for media consumption, you can't go wrong with Kindle.
  https://www.amazon.de/7-Tablet-Alexa-Display-Schwarz-Spezialangeboten/dp/B01J90P010/ref=sr_1_10_sspa?ie=UTF8&qid=1534686031&sr=8-10-spons&keywords=kindle+fire&psc=1
  (the link is for more versatile Kindle Fire, but if it's just for reading, getting Kindle Paperwhite would be a better option)

• USB fan
  In case something goes w rong with air conditioning. It won't replace a full size fan, but it can keep your face comfortably cool long enough, until the real cooling is back up and running. Different variations are available. For smartphones, for laptops, and even portable handheld fans (charged via USB).
  https://www.ebay.de/itm/172573365285
  (for me, laptop fan is the best, as it's significantly more powerful than smartphone fans)

• Waterproof bluetooth speaker
  If you're more of a party animal, and like to have some music on your holidays, you'll need a bluetooth speaker. But there is a good chance that at some point you will get drunk, and your friends will be drunk, and you will be at the beach, or at the lakeside, or near the pool. Then if you didn't buy a waterproof speaker, you'll surely regret it.
  https://www.ebay.de/itm/362395056545
  (expect waterproofness, but manage your expectations with sound quality)

Cool, but rather optional:

• Sports camera
  Not necessarily the most expensive model with all the bells and whistles. But water resistance is a must. Unless you are a professional, I would also recommend relying on software image image stabilization, and using it without any external stabilizers.
  https://www.ebay.de/itm/223106144902
  (Link to Xiaomi Yi Action Camera, providing great value for money when it comes to scpecs, and decent level of waterproofness)

• Solar charger
  Solar Paper is the only reasonably portable solar charger. It's quite slow, and requires direct sunlight in order to work properly. However if you are a survival maniac with a lot of money, that's the way to go.
  https://www.amazon.com/Paper-thin-Portable-Charger-Ultra-High-Efficiency-5W/dp/B01GNF777Y/ref=as_li_ss_tl?&ie=UTF8&linkId=6c2edb848061b0101ebd7d2bcc70ecad
  (unavailable on eBay, watch out for counterfeits)

• Cheap quadcopter
  You don't need actual drone (with GPS positioning, ability to operate independently, etc.) in order to have some nice photos taken from above. A cheap quad-copter for 10€ will do the trick just fine. If you're on a tight budget and don't already have a sports camera, consider buying a model with the camera built-in.
  https://www.ebay.de/itm/253788443608
  (shipping from Israel, no customs)

• Disposable phone and disposable camera
  Do you know yourself too well? Are you expecting to get drunk every day during your trip and lose or destroy your phone eventually? Consider buying cheap entry-level phone, and entry-level digital camera. You will still get ability to use social media, and ability to shoot some moderately decent photos. And even if you lose one of those, you still get to keep the other one. It sounds crazy, but too many people lost their 1000€ phones on holidays.
  Phone: https://www.ebay.de/p/5024284050?iid=153055515958
  (Kenxinda V5 for 30€ - Facebook and basic apps are quite usable, although I wouldn't try multitasking)
  Camera: https://www.ebay.de/p/222719100?iid=263847673519
  (Nikon Coolpix L31 for 40€ - dubbed 'too cheap for its own good' by reviewers. Even at MSRP of 70€ provides better image quality than many cameras that are twice as expensive)