What next?

I've finished singe variable calculus and I want a taste of pure mathematics before I go to university next September. I'm going to get a grasp of sets and proof first but what next? Number theory, analysis or what? Book recommendations would be ideal!

We need engineers more than we need mathematicians.  If I asked a mathematician to build a sewer plant, it would cost 10 times as much, and always be half-finished.