I don't know much about weddings and what would constitute a good place for that but I live in Costa Rica right now so can answer almost anything else.
Here's my basic recommendation for Costa Rica:
1) Anything worth doing is outside the capital (San Jose). There's really only casinos as a tourist attraction in the city.
2) The tourist stuff all tends to be ecotourism, so it's a good spot to come to see nature but don't expect a lot of cultural tourism.
3) The nicest spots I've been to here are: Manuel Antonio and Arenal and Guanacaste is also supposed to be beautiful but I haven't yet been there.
So here's my recommended itinerary based off what I've done here:
Manuel Antonio is an ecological preserve, with a nice tourist town and lots of rainforesty stuff (you might run into a monkey on the beach or something). Stay in the hotel Si Como No ( http://www.sicomono.com/
) and go to the beach inside the reserve (much more secluded and beautiful).
Arenal is a volcano around which are spas and resorts. One in particular has some great hotsprings where you can sit and watch lava flow down the volcano. That's the main attraction of the whole area (even if people stay in other hotels they buy passes to go into this resort's hot springs) so you should make sure to stay at that hotel if you go to this area. It's called Tabacon ( http://www.tabacon.com/
Other things to do include Sport Fishing, and canopy tours (zip line through forest canopy) but I don't have a lot of experience with that since I tend to go more local than touristy. You should be able to plan that kind of thing with each hotel though, and I can find out more information on that if you'd like.
Guanacaste is up north, and is a much hotter and drier climate, but from what I've heard has the most beautiful beaches (the beaches themselves aren't very pretty here, but the greenery is). I don't have any specific recommendations but have friends who have houses there or who live there so can ask if you are interested.
If you have any specific questions let me know.