I've been bringing in some serious side $$ by doing weekday dog walks for busy working people. If you live in a HCOLA or even MCOLA, a lot of people are willing to pay a lot of money to have their dogs walked while they're at work.
I live in a HCOLA. I put an ad on craiglist as a dog walker, and had a few responses within a day... The great thing is I'd go out walking my own dog every day anyway, but this way I make money and can't ever cop out of my daily walk. I charge $15 for an hour of walking, and I think that's on the low side for that amount of time. At two dogs per weekday, that's a nice $600 cash per month. I also pick up and drop off.
I've seen people out with 5-7 dogs. That's too many for me, but if you have the physical strength, that's a lot of cash!
I've done a little in-home pet care as well, and have considered signing up for rover.com, but not sure I'm that committed just yet. $50/night. Cash. Old sleepy dogs are best. :-)