Hi S.,
I know how you feel leaving your precious cargo with a stranger. About paying your take-along babysitter... I don't know how old your babysitter is, but I would think if you're providing her with (esentially) a free vacation in Florida, that would be payment enough. If you are paying for the gas down there and back, meals, room, etc., seems to me that you've compensated her plenty. Even if she's paying for her own room and food, but has lots of free time at the beach, or wherever, it's still a good deal. The only other thing (assuming she's old enough) that I could think of is her own rental car once you got there so she could go places. Or if she's young enough and you have the room, allowing her to bring a friend with her. (Just brainstorming here).
In regards to entertaining your son, I don't know how you feel about TV viewing, but my daughter who is also 22 mo., watches a little TV during the week. The longest trip we've taken so far has only been 5.5-6 hours, but she LOVES the portable DVD player and a Disney-type movie, but of course, she IS a captive audience. We put it in when we leave, and make pit stops to change her diaper and the movie. She'll usually go to sleep during one of them, but it keeps her from sleeping the whole trip (and being wide-eyed and wired upon arriving at our destination).
I don't know if your son is interested in markers, pens, and crayons, but another option might be a tray on the car seat (or back of the seat in front of him) and those wonderful Crayola Color Wonders markers that only mark on the special paper.
I don't know if boys bore more easily than girls, but that's what we do with our daughter.
G.