How good a surfer are you? Do you prefer beach or rocky point breaks, lefts and/or rights?
Jaco has the most mongering options if you want to mix "sports" but the best surfing is outside of town, which sounds like it would be a problem for you. The beach in town is pretty yucky and the waves very undependable.
There are some very famous surf spots in CR. Witch's Rock at Playa Naranjo is probably the most famous (strongest waves during the Dec.-March windy season), but it is very remote with no facillities (ie bring your tent). I believe they have tours that go out there from Tamarindo, but those would not be for the budget minded. Pavones on the FAR south Pacific coast has an excellent left point, considered one of the longest in the world (like over a mile!). Good shape and very fast. But it doesn't always run and it is 8hrs by bus from SJ. Puerto Viejo (4 hrs from SJ) has its famous "Salsa Brava" right offshore in town, a very thick and voluminous wave that comes from deep water onto a shallow reef. This place gets very big and TUBULAR (ie REAL waves and probably not for beginners, unless they want to risk getting scraped up on that reef).
Of those, I think Puerto Viejo may be your best bet for cheap, easily accessible places with surfing in or very close to town IF you're up for it. If not there are also good beach breaks just south of town. The other good candidates would be the Playa Dominical area (good, strong beach break with lefts and rights right in town and lots of cheap places to stay) and the areas around the southern tip of the Nicoya Peninsula (Sta. Teresa or Mal Pais - beach and some point breaks, with very consistent lefts and rights and also lots of cheap places to stay).
Here is a good overview map of the different surfing spots:
http://www.centralamerica.com/cr/surf/surfmap.htm and another link to a site with good surfing info:
http://www.wannasurf.com/spot/Central_America/Costa_Rica/index.html