Costa Rica: Waterfalls and rivers

I'm planning my next trip to Costa Rica and would like to see several
waterfalls, and perhaps raft on a whitewater river. I'll most likely be going
in February.

Anyone have any recommendations? Falls where one can (safely) hike
around/near/behind are preferred to ones I can just look at briefly from the
road, but I'd be interested in any nice ones that you know of. I briefly saw
one on the road from Arenal to Alajuela, and was entranced.

Last time, I took charter vans between the areas I was staying. This time I'm
considering renting a 4x4 and driving myself, though I must admit to a fair
amount of anxiety about getting lost due to the lack of road signs.

I'd like to explore the Meseta Central and other areas as well, and as I said do
a Class III or so river with a reputable river guide in one of those big
inflatable rafts, perhaps even an overnighter as I'll be carrying just one
backpack.

Any ideas?