My senior year I scaled back my studies to < 10 hours a week (including class time and papers) and spent 60+ hours a week working on a startup. It was absolutely the right decision. I had my expenses paid for, got to live with all my friends, yet still had lots of time to work on my startup.

The only thing I might suggest is that if you don't have a clear idea, you might want to consult for a while in the target industry. That way you can learn the industry, meet the right people, and understand what people will pay money for.