I'll second the mousesavers suggestion.
As for the booster seat, I'd take one from home. It costs about $10 per day to rent a car seat/ booster, and you never know what you're going to get. If you child is big enough for a low back booster, that can be packed in a duffle bag surrounded by clothes. If I were bringing a high back booster, I'd probably carry it through the airport and gate check it.