I've done summer programs in Ecuador (
www.latinoschools.com/) and Costa Rica (
http://www.interculturacostarica.com/). You can do from as few as 2 weeks to the entire summer. Both are about $300-350/week, and have immersion classes for 4 hours a day, and include a homestay with 2 meals a day. The school in Ecuador offers college-level classes, and credit, but the school in Costa Rica has many more cultural activities for free.
I really enjoyed both, but in terms of improving Spanish, you really have to make an effort, because it is really easy to just go through the entire program just speaking English to the other Americans in the program.