Er, island? Was very firmly attached to Nicaragua and Panama when I was there a while ago! We travelled for a couple of months and stuck to the Highlands and the Pacific coast – really enjoyed it, though it's probably changed quite a bit. We stayed in Manuel Antonio, which was just becoming popular and then travelled down to the Osa Peninsula – remote and rainforesty. The surfing is excellent.
But it's not a cheap place, even as a backpacker. Think American prices. If the budget is an issue, what about Guatemala? Lots of interesting Mayan ruins. I hesitate to recommend Mexico these days, but the south is probably fine and again very interesting. Costa Rica is more about the jungle, 'eco-tourism' and beaches. I would also imagine it to be much more resortified now.