Hamburg's Alster is not just the big lake in the middle of the city - it spreads all over the city via various canals. You can rent boats, bring some wine, bread, cheese, and see the city from the water.
I found a company - Bootshaus Silwar - that hires out canoes (note that I am not affiliated with them nor know if they are the best and cheapest or anything, but it should help a bit).
Eppendorfer Landstraße 148b;
tel: 040 47 62 07;
email: info@bootshaus-silwar.com;
www.bootshaus-silwar.com