I'm learning Spanish as my second language, so I won't have time to study Mandarin Chinese in school. I don't have time for exchange programs either, because I'm focussed on going to a Spanish-speaking country. Any suggestions? Any textbook recommendations?

Try Rosetta Stone

It is difficult to learn 2 languages at the same time. You will get mixed up between the two and waste a lot of time. Concentrate on Spanish. Mandarin can come later once you master Spanish.

Well Mandarin is a very difficult language to learn, especially if you're doing it by yourself! That means you have to dedicate yourself to it 100%. It sounds like you're already too busy with Spanish to have all that dedicating to Mandarin. In any case, I always think penpals are the best way to really learn a language :) Rosetta Stone, also, browse around for the best books, also, for beginners :)