Another option that I think is nice is dog/house sitting (never done this but we use one). I think we pay about $50 night and the agency takes a cut (not sure how much). But all you have to do is walk the dog/feed it/etc.
Honestly, I have thought about this, but accepting 1 or 2 dogs in my home instead of driving around.
I'd look into that carefully. Taking other dogs into your home for money (even 1 or 2 at a time) could be construed as running a kennel. That's not allowed in many places without zoning approval, which can be very difficult to get in some places; it all depends on local politics. Taking care of a dog in its own home is different.