Jaco offers the perfect rainforest ATV options. Take your ATV 15 minutes and you are in deep mountain rainforest. There are two hour tours that take you to waterfalls, rainforest and the beach for around $55 for a two hour rainforest tour.
There are a few options of places to go with your ATV tour - waterfalls, rivers or just go into the rainforest. It is well worth it.