I've gotten minivans from bot Hertz and Thrifty at LAX. Both were in October, and prices seemed to hit their lowest about 2 weeks out, but it's entirely dependent on demand-- I paid less than $200 total for one of them (in 2011) and maybe $250 for the other (in 2012), both times for a full week. Usually what I do is get at least one reservation in as soon as I know I'll need it, then keep checking back to see if prices change. I live in LA so I shuttle down with someone and pick up the car, so I can't speak much to convenience other than that they all either pick up at the airport or at the Lot C cellphone lot (which you can get to on a free bus).