This is dumb as shit ofc, but it gave me an idea that's probably nearly equally dumb as shit:
Regular bicycle, but with an extra gear that can selectively connect to the chain or wheel or w/e, that's connected to a coil torsion spring on a kind of ratchet release.
Basically you flip the switch when it's a good time to rob some energy like when you're on level ground or going down hill. That energy makes you a tad less efficient (but you don't care cuz it's level or downhill), and uses that energy to wind up the coil torsion spring up until a max amount of torque is stored.
Fast forward a bit: now you're approaching an incline, so you flip the switch the other direction and that torsion spring regurgitates that energy back into forward motion, giving you a nice forward burst when going up a hill.
Not free energy by any stretch, but a strategic use of what you're already spending.
Feel free to explain why this is a horrible idea - I'm about as far from a physicist as it gets.