What do your friends and family do? Find someone you know who is really good at what they do, in a non-programming field. Ask about their workflow, and try to find the areas where they could have better tools. Then start to build those tools.

The most innovative and useful ideas seem to come from a mix of high level domain expertise, and high level technical ability. If you don't have both of those, find someone who has the piece you are missing.

Edit: sorry, assumed you were op.

Those sound like great hobbies to base your idea on. Out of the two, I think Tennis would have more of a market (aka make more money). Just throwing out some random aspects/components of Tennis that you could bounce around; the game itself, the equipment/clothing, the training/education, the facilities where people play, the professional sports aspect, sponsors and marketing. Some of the people involved; the players themselves, the people that run the facilities, the people that hire tennis tutors/teachers (and the students), the people that watch the sport professionally, the people that sponsor the sport, the people that provide equipment/clothing. I do not have much experience/knowledge with Tennis, however my Cousin's Husband is a Tennis Instructor, so I'll go with that. One of the problems that he has is finding more students to teach and fill up all his potential slots. One of the problems that Tennis Students have is finding a good teacher with a reasonable price. Going off this, maybe one potential idea is to build a "Marketplace" for Tennis Instructors. Call it Yelp for Tennis Tutors. So instructors could post up their Business with their contact information, Students could review them, and you facilitate the connection of Teachers and Students. You're solving multiple problems. The Teachers find more Students, the Students find good teachers, and the best price. Your Business Model could be a re-occurring monthly revenue model that Teachers pay to add their Business to the Marketplace. A secondary Business Model (or replacement) could be a one time fee per transaction every time you facilitate a connection between a Teacher and Student (as in, the Student "Signs Up" for their Services). Anyway, just throwing out some ideas! Hope I've helped with some brainstorming!