Compost - even though you don't have a garden, you can compost them for container plants. (double check on possible acidity of the rinds first)
Fragrance - the rinds can be simmered in a mini crockpot for a nice fragrance.
Cleaner - I make my own all-purpose cleaner and lemons come in handy. Use the cooled off simmered water from above in your homemade or prepared cleaners.
Rough elbows - To soften and lighten elbows you're supposed to use the actual lemon. As a twist, use the scooped half rinds to hold soaking liquid for your elbows.
Garbage disposal - Freshen your disposal by grinding up the rinds.