And don't knock it unless you've tried it! pretty much every landscaping customer I've ever had has also had pets... its amazing how one can combine the petcare with the landcare.
I charge $30.00 an hour for visits.. so its not bad. and much easier than pushing snow.. though it can get dangerous at times too.
my brother moved from IN to Virginia for a teaching job. Towards thanksgiving he placed an ad on the net for pet sitting over the week-end. He ended up with almost 20 jobs. Some just to let out and feed, others he had to take to the park and play with.
His charges were about the same as yours. $25 per day to feed and let out. $50 an hour to take to the park or for walks. He never actually took the pets home to watch. He also made sure they knew he was just looking for extra money for the holidays and wasn't in the business full time, so he didn't have the ins. and all the legalities.