If it were only a week or 10 days, what I'd do is to find a quiet residential area and park on the street. It wouldn't be unusual for a vehicle to be parked somewhere, and not go anywhere, for that length of time. When I lived in Edmonton during the week a couple years ago, I'd park my car outside the house where I was renting a room on Monday, and wouldn't touch it again till Friday ... no problem at all.

3 weeks might be a bit long for something like that. The neighbours might start questioning why this vehicle is there. I think if I were leaving my car somewhere for that length of time, I'd inquire at churches.

Whatever you do, don't ask to park in a bicycle shop's parking lot. From my experience, most bicycle shops don't have a whole lot of parking available, and what what they've got free for customer use.