I always have a few around for making spare parts... (heating and stretching them to make an antenna, sanding them and carving them to replace some brace for osmething) it always comes in handy...
not a bad idea about the go cart... never thought of that.. you could practice heating and strecthing them to make them rounder and more tubular like a go kart, heck.... you could even bend the roll cage and stuff out of it too! go for it! I'd like to see pics!