If you are really serious about learning Spanish, complete immersion is undoubtably the way to go. When you stay with a host family that speaks no English you have little choice but to try. It may be daunting at first, but whomever you speak to will generally offer encouragement and will complement you on your Spanish, even though you might think it's not very good. Finding your way around by riding the San Jose bus routes is an adventure in itself, and if you get lost you can always find a taxi to take you back home.
Seeing how other cultures live and how they've learned to adapt to problems unique to them gives you an appreciation of their solutions and broadens your mind as to their uniqueness. I can't speak about other Latin American cultures, but I can say that Ticos are some of the most welcoming people I've ever met.
You will be overawed by Costa Rica's countryside. From beaches to wildlife to active volcanoes, get ready to be wowed by the sights of Poaz, Arenal, and other volcanoes. The beach and jungle wild life in Manuel Antonio is not to be missed.