Since your time is limited, be very careful with your timing at the
Purple Pig, which doesn't accept reservations. You can encounter waits of an hour to be seated at lunchtime, and two hours is not uncommon during the dinner hours. If you can break away from your conference at non meal times - for example, during the afternoon between roughly 1:30 and 4:30 - you can find immediate seating.
There are some other excellent places nearby, which don't involve long waits.
Sable, a couple of blocks from the Marriott, is one of my favorites; it offers contemporary American food in a small plates format. Also close by are
Emilio's Sol y Nieve for tapas,
Coco Pazzo Café for Italian (although personally for Italian I'd walk the few extra blocks to
Piccolo Sogno Due),
Shaw's for seafood,
TRU for fine dining (dinner only),
Quartino for Italian small plates, and
Eataly for everything Italian. I'd also recommend
Frontera Grill and Topolobampo for contemporary American; you can now make reservations for Frontera on
cityeats.com. All of these are within 2-4 blocks; extend your distance to 6-8 blocks, and you can hit
GT Fish & Oyster for seafood small plates,
Brindille for French,
Naha for finer dining, and
Meli Café on Wells for breakfast. Also the original
Pizzeria Uno and Pizzeria Due, and
Lou Malnati's on Wells, for our delicious deep-dish pizza. Note that Lou Malnati's lets you phone ahead with your pizza order so they'll have it ready for you when you arrive and you don't have to wait 30-45 minutes while seated for your pizza to bake, but I don't think Uno and Due offer that service.
With the exception of the pizza places, most of these others accept reservations on Opentable, and reservations may be a good idea if you're dining during the busy lunch or dinner time slots.
You may want to check distances, as some of the other recommendations already posted here are in the far end of the Loop or towards the north end of the Mag Mile, so you may be looking at a 10-20 minutes walk. Maybe that's okay, but it sounds like you want to keep it closer to the Marriott.
Other places I'd recommend which are not full restaurants include
Toni Patisserie for French breads and pastries (MUCH better than Hendrickxx IMHO),
Intelligentsia Coffee on Randolph for coffee,
Garrett Popcorn on Michigan for caramel and cheese popcorn, and
Glazed and Infused on East Hubbard,
Firecakes on West Hubbard, and
Do-Rite in the Loop for donuts.
Also, if you get the time, a trip to the
French Market may be worthwhile, even though it's about a mile away. It's a public market which has booths for some of our very best restaurants and food stores of their kind in the city, including Pastoral for cheese and sandwiches, Fumare for Montreal-style smoked meat, Vanille Patisserie for French breads and pastries, Lillie's Q for barbecue, and Saigon Sisters for banh mi and pho.
You mention Thai restaurants. Be forewarned, the ones near the Mag Mile are nothing special. If you want good Thai cuisine, you'll want to consider taking the time to travel to the neighborhoods where you'll find places like
Andy's Thai Kitchen and
Rainbow Thai Cuisine. If you can't take the time, I think you'd be better off doing some of these other non-Thai places that are close by, rather than the Thai places near the Mag Mile.