Backstory: Both Julia and I have bikes that we use once a week or so. When we're not working, why not let someone rent out and ride our bike around the city?
The Idea: A bike rental/sharing platform
Deep Dive: You could start by building a simple marketplace that costs a fix amount to post a bike and upon a successful match, the buyer and seller are required to transfer funds and coordinate the bike handoff.
As this takes off, you could build a locking mechanism that can remotely unlock the bike for the buyer. Additionally the lock could be used as a gps tracker to help monitor and follow the bike.
Validation: I've seen people on our local neighborhood facebook page asking to borrow someone's bike for the day. You could make a simple test by posting your bike on craigslist and sending over a lock code to see if it works. Beware that person may steal it but I have faith in humanity :)
Monetization: In the beginning you could charge a flat fee and experiment with taking a percentage of the transaction.