If you're talking about just downtown, it's easy to do a coupon run on foot, and it'll be generally safe. Just watch your back when you wander off Fremont or up to the El Cortez. You can plot a large oval, beginning and ending at the Plaza or the EC.
At no point will you have to go more than one block from Fremont, no matter how you do your route. I'd do it in the evening, when it's cooler, all the lights are lit, and the FSE is blaring away. Enjoy the human freak show.