Khao Yai National Park
Thailand's oldest national park, and one of it's biggest, seems like it would be a great spot for hiking. I've only had the pleasure of an afternoon visit, but the scenery is great and the place is packed with wildlife; while we barely left the road, we spotted monitor lizards, elephants, monkeys and plenty of birdlife.
Park entrance is officially 400 baht for foreigners these days, but they still seem to be charging the (still excessive) old price of 200 baht.
Leeches are a problem here, especially in the wet season; bring long pants, or you can buy leg covers for 50 baht at the information center.
There are plenty of camping spots inside as well as some proper accommodation; ask at the information centre for more details. They also arrange hiking, bikes, bird watching trips etc. for some very reasonable rates sometimes.
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