There are a couple things I can think of...
1. Get in with a car/boat/motorcycle storage facility to provide detailing over the winter months while the vehicles are in storage. It offers an instant client pool that you have access to. You only have to move your equipment and products in and out once. Plenty of these places are heated. The people who utilize these facilities are likely to want their vehicles looking good. After the detailing mid-winter, they will probably only need a waterless wash, LSP and dressing before being picked up in the spring.
2. Paintless dent repair. This has been talked about a lot here and the intro cost is pretty steep.
3. Wheel restoration. A lot of people have dedicated winter and summer sets, and they always want their summer set looking perfect. The wheels are already off the car. It's a good option for people who don't want to drop $500+ to have their wheels powder coated or only want one wheel refinished. The set up and materials are cheap. This would be an easy place to up-sell a wheel coating like CQ or OC. Only trouble would be finding a heated space for the paint work.