I would be wary of Frontera Grill. As others have mentioned, the food is great, but it's cramped and noisy. The bigger downside is that they don't accept reservations in advance, although they take them the same day you're eating if you call them when they first start taking calls at 8:30 a.m. That makes it a bit of a crapshoot when it comes to advance planning. If you're really interested in Rick Bayless's regional Mexican cuisine, you might want to consider Topolobampo next door, where they DO take reservations in advance. Other alternatives in the regional Mexican genre include
Adobo Grill in Old Town and Wicker Park, and
Salpicon in Old Town.
MK is another place that I would be wary of. Like Frontera Grill, it's crowded and noisy, and the last time I ate there, we felt incredibly rushed (we joked that the name stands for "mangia kwick").
I haven't been to
Kevin.
For something fun downtown in a casual, hip, contemporary American vein (a la MK), my recommendations would be
one sixtyblue,
Blackbird,
Aigre Doux, or
Naha. To throw out a few more kinds of food, you might consider
Red Light for Asian Fusion,
Bistro 110 or
Kiki's for French Bistro,
Cafe Spiaggia or
Coco Pazzo for Italian, or, if cabbing to the South Loop isn't too inconvenient,
Cuatro for Latin Fusion. All of these offer excellent food in a fun, casual setting.
All of the restaurants mentioned here (other than Frontera and Kevin) accept reservations on
opentable.com as well as by calling the restaurant. (Kevin accepts reservations by phone.)