Do the summer immersion thang, but have the equivalent of at least one semester if not one year of college level Spanish.
For someone of average linguistic ability and no background in Spanish, a full time immersion program for three months in Spanish will give you the ability to speak comfortably in simple present, past and future tenses and nail down the regular conjugation of lots o' verbs. Conditional and perfect forms will be dealt with but not to the point of second nature. You will be able to be comfortable with basic conversations, but nothing very complicated.
If you learn these basics BEFORE going down, you get a big boost. You will be able to spend your time with verbal fluency, so that by the end of the summer, you'll have the basics down to the point you don't have to think about it. You'll have the ability to learn about twice as many verbs while you're down there. You'll know your perfect tenses and build enough vocab to talk around things you don't know offhand.
To gain basic ability, Pimsleur is probably the best book/CD series. For software, it's very hard to beat Rosetta Stone.
