It's a little off the beaten path, but Santiago's on Route 27 in Clermont has GREAT Mexican, and is very inexpensive (a family of four could eat for under $40).
It's about a 20 minute drive from WDW, go west on 192, turn onto 27 North and drive about 15 miles (65 mph speed limit pretty much the entire way), it's located at the base of the Citrus Tower. If it's still light out you can take the elevator to the top of the Citrus Tower for a scenic view at a small fee.
Anne