If you have unused garden space, consider renting it out for someone to use as a vegetable garden. Ideally, you would have enough extra space for a greenhouse. Place an ad in local supermarkets for someone in your local area.
If you have a decent sized garden and live near a tourist attraction you can rent out garden space as a mini campsite. How much you can earn depends on where you live and the facilities you can offer.
CampingInMyGarden.com is a website where members can advertise their garden space and campers can search for suitable garden pitches.