I was there. Khao Yai National Park

Aside from feeling discriminated for paying
400 baht instead of the 40 bhat that Thai
people pay (even if you're working, living
or whatever in Thailand, just because
you're from another country you pay 10
times more!), the park is beautiful and well
maintained. You can see some animals,
flora and fauna and breathe clean air.
The ideal thing is to spend a night there
and do some excursions and avoid the
holidays and the weekend. There are some
restaurants inside the park, toilets and you
can even get gas for the motorcycle in case
of emergency.