I go back at least once a year and will be there in May. It's changed a lot since I left in '93 and has become much more metropolitan. There are outlet malls but will have to check on the locations. Everything does seem like a bargain so I usually just go to the regular shops. I would say especially trainers and children's sandals are very good value (I always buy DS's when we're there). Of course, American shops like Gap, Calvin Klein, etc are cheaper than in the UK and there's a shop called Old Navy which is owned by Gap and is an even more discount version. Great for basics like Tshirts and shorts for the kids.
I don't know what you're into but US brand beauty products are cheaper there too and you MUST go to a big drugstore (chemist). Eckerds or Walgreens are my favourites for all kinds of weird and wonderful stuff. I recommend checking out a grocery store too. Great cake mixes and Mexican products. Mexican extract of vanilla is the best in the world but not sure if it's legal for customs due to alcohol content. For some reason American cinnamon always tastes better to me too and I bring some back with me most trips.
Other great stores are Pottery Barn for home stuff, Restoration Hardware for more home stuff, Sees Candies for pressies for friends, Bath & Body Works for yummy smelling bath products, Victoria's Secret for great undies (both practical and sexy styles). And, of course, if you want genuine Western apparel there are some great boot makers, cowboy hat manufacturers, huge belt buckles, etc. The World Championship Rodeo is in Mesquite, where I grew up, and could be a fun night out. I should warn you though that if you're a great animal lover things like calf roping can be a bit distressing. Not my thing but an interesting cultural experience.
March should be really nice. Lovely sunny days and cool nights. You'll probably be there during Spring Break when the schools are out so there should be lots of organised activities at all the museums and tourist sites.
Hope that helps!