Where would you go if you had a full tank and nowhere else to be?

One tank of gas from flagstaff? you might make it to Saint George, but you'd have to walk home.

If you are looking for driveable destinations, this is largely dependent on what you want to do, more info would help.

As a catch-all, Las Vegas has it all and is a very reasonable drive (almost too reasonable for a spring break trip, as you can probably do weekends there if you want).