I can second Trattoria Roma for seemingly-authentic Italian bstro food; Adobo Grill is close/convenient, but I think Salpicon is better/less touristy for Mexican if you don't mind walking a few blocks.
Bistro Margot has its detractors, but I've always found it to be a reasonably good bet for French bistro food. It's been discussed here:
http://www.lthforum.com/bb/viewtopic.ph ... 0d74c0884f
I've been thoroughly underwhemled by Mizu - seems like the restaurant is all about the Yakitori as a differentiating gimmick, rather than a true attempt to serve up great food. It can be fun, though.
For Middle Eastern,, Old Jerusalem has been around seemingly forever (it was there when I lived in Old Town in the 70s and 80s), and was still reliable last time I was there - much more casual than the others, though.
Trattoria Roma
1535 N. Wells St
Salpicon
1252 N. Wells St
Bistrot Margot
1437 N. Wells St
Mizu Yakitori & Sushi Lounge
315 W. North
Old Jerusalem
1411 N. Wells St