Find a repair that you need done on the car (new struts, for example), contact a chain repair shop in the area, say that you want this done and can you leave the car there for two days while they do it? Check on it, but if there's a problem with the repair, you can typically go to the chain location in your area to have it fixed (instead of having to go back to the exact place that did the work). The security at those places is typically quite acceptable, and you'll save time by having your car fixed while you're on vacation.
Otherwise, wherever you park it, it's a good idea to ask first so that somebody keeps an eye on the car and prevents it from getting towed, but usually a couple of days isn't a problem.