This past June I did a short one-week Spanish immersion course: me, the wife, and the (reluctant) 6-year-old. I can’t say I started suddenly dreaming in Spanish or anything from five days of instruction, but as a learning vacation it was quite useful.
Budget Travel has a great article from Joshua Berman on studying Spanish in Nicaragua. Joshua has written for Perceptive Travel before and he’s the author of several Moon handbooks on Nicaragua and Belize.
There are some advantages to studying someplace like Nicaragua instead of where I did in the main tourist city of Guatemala. You’re forced to stay away from English more often and many students feel like it’s more personal than it is in a place where a whole industry has built up around the idea. Then there is the tried and true advice that many have used to become fluent: get a local girlfriend/boyfriend.