I think Chili's is one of the best chains for eating healthy... Here's a good link to a site called healthydiningfinder that has info for lots of different restaurants. It will even include options where they are good if you special order...
http://healthydiningfinder.com/restaurant/?id=62
the Carribean salad would be good if you can do without the dressing.
The margarita grilled chicken is a good option, depending on the sides you pick. The fajitas are not a bad choice if you request no cheese or sourcream and use only 1-2 flour tortillas.
And they have a section called guilt-less grill - there are sandwiches, wraps and entree plates served with extra veggies specifically to be healthy...GG Grilled Chicken Sandwich w/ Steamed Broccoli, GG Santa Fe Chicken Wrap w/ Steamed Broccoli, GG Salmon w/ Garlic & Herbs, GG Classic Sirloin.
Everything I listed would be under 600 calories and a few are under 500.