This isn't going to be much help, but we honestly do anything we can to avoid having to do naps anywhere but home. The only exception to this is plane travel, where we're sure he'll sleep, until recently b/c he would bf to sleep on takeoff, now he'll still go to sleep b/c he's being held.
I have a friend with a ds the same age, and they often go out and about all day on the weekend. Her ds will usually nap in the pushchair, although often not for more than 45 minutes, but they just deal with the cranky toddler that results...