good mexican and fun - westside?
Monte Alban on Santa Monica is great Oaxacan, and a great deal. I also like Border Grill a lot, but it's more pricey - and you make sure you make a reservation there. Haven't been yet, but I hear that Don Antonio's on Pico is pretty good too - and certainly an interesting atmosphere.
It's not "fun", unless you're 50+ maybe, but my favorite Mexican on the westside is Tlapazola Grill on Gateway.
Lares on Pico used to be high on my list, but since they reopened after the fire over a year ago, the food has been inconsistent. Still, it's worth a shot. When they're on, their carnitas are some of the best I've had, as is their chile verde burrito.
You could also try Casa Escobar on Wilshire near 26th street - it's usually pretty hoppin' on the weekends with a younger crowd. Food is OK, but very potent margaritas. Good crunchy tacos, tostadas, and a very respectable Chile Colorado burrito. Stay away from the tamales, as they are bone dry.
My recent fave is The Talpa on Pico w. of Barrington. Great food across the board. Not much ambience, but I go for the food, not the digs. Nice spicy table salsa and thick crunchy chips.