I have an 11 year old Cannondale M500 that is perfect for errands and even commuting. I would hate to lose it mostly for sentimental reasons and am pretty careful where I park it. I lived in a ghetto area of downtown for awhile and parked it outside stores and the gym and no one seemed to tamper with it. I would check on it periodically through the window of the gym. An hour is a long time for her to be outside alone in a strange place! Now I'm in the suburbs (ugh!) and feel pretty comfortable leaving it out wherever I go... locked up, of course.
I did use a 70's model Schwinn cruiser for awhile that was a hand-me-down from my mother. But I fixed it up and made it work well and look good... so my mother asked for it back! HA! I was glad to give it to her 'cause I know she'll get more exercise with it. "Everyone in town rides them to the store and such so I need it back," she said. Guess I should have come back with, "If all the retirees were jumping off cliffs, would you follow?" I've got my eye on my grandfathers even older cruiser bike... already in great condition... with basket!!
Heck, if you're not going to be doing jumps or running stairs, check out the used section of your lbs or a pawn shop and get a decent bike for a decent price that you won't have to worry about. Another suggestion I've heard is to take all the brand stickers off and put a couple of your own stickers on so that it looks a little trashy. The parts hounds won't waste time on what they think is a junker.