I've spent a few weeks in Costa Rica and about one week in Nicaragua. CR is hot, lots of jungle, and is pretty well developed. It's also about as safe as Central America gets. I travelled a bit around the place, went to see the volcano at La Fortuna, but spent most of my time in San Jose and the Tortuguero national park where i was (nominally) volunteering.
I only went to San Juan Del Sur in Nicaragua, when i had to leave CR to get my visa re-done. It's a backpacker town, which i like. Nice beach, cheap hostels, surfers, that kind of vibe. It's not yet like Khao San road, but it will be one day I expect.
Rum and coke (both kinds) are cheap and plentiful.
There's a backpacker trail all through latin America. They call it the Gringo Trail. You'll find decent hostels etc and all the tourist attractions on the trail, and you'll be safe. Obviously there's some parts of latin America best avoided, but the same can be said for anywhere really.
I spent most of a year travelling solo from those countries down through South America, so if there's anything specific you need to know, just ask. If I was going back over there though it wouldn't be Nicaragua or CR, it'd be Colombia or Argentina, as I liked them better.