Hamburg city has a good bike system – just swipe your credit card to set up an account (fairly simple to do) and then take off with one of the red bikes.
The pricing guide wasn’t completely clear at the terminals, but basically the first 30mins is free before it starts charging. Don’t keep the bikes locked up, the charges get higher and higher the longer you keep it away from a bike drop. There are plenty around and we didn’t have trouble. The only trouble we had was sometimes the bike drops were full and you’d have to zip around the corner to another nearby.
The city is flat and there are plenty of bike paths so it’s a simple way to get around. See post XX for suburb to visit. Cycling around the city lake is also a good idea, and there are plenty of cafe on the path or nearby to stop at on the way around.