There are literally hundreds of Mexican restaurants in Denver, some excellent, some not so good. A real authentic place, with great food, is El Taco de Mexico. It's a lunch counter kind of place and serves things like brain and tongue tacos. But there chili relleno burrito is not to be missed. They have a pretty good menu variety and most of it is excellent. It's also centrally located and easy to get to. It's at 7th. Ave. and Santa Fe Dr.
There is another place called El Noa Noa right next door where the food used to be some of the best in the city, but it seems to have lapsed in the last few years. You may want to give it a try, though, if you're looking for more mainstream mex food.
Another great place is further out at 7035 E. 72nd. ave in Commerce City. It's called La Casa Del Rey and is a family owned joint with a huge menu, all of it good. To get to it, take Colorado Blvd. north to 72nd, then go east. It's out in the boondocks but worth the trip.
Brothers is a great place for BBQ. It's loctaed at Leetsdale and Monaco in Glendale, and is also pretty easy to get to. Excellent 'Q' in a town not known for it.
Boulder, being a college town, has lots of funky places to get decent grub. Try the Pearl St. mall. There is a cafe I like on the mall at 17th. St. called The Sunflower. It's more upscale, though, and deffinetly not road food, but the food is all natural and probably near the best in Boulder. There are many other decent places on the mall, including a couple of excellent pizza joints. Another good pizza place is Beau Jo's, a small local chain, where you can get an original "Mountain Pie". I think it's on 28th st.
If I come up with any other ideas, I'll post 'em here. [
Hope you have fun on your trip!