Suggestions of good restaurants and fun things o see and do
Catch a sporting event to get the full Chicago sports fan experience (basketball or baseball if it's going on), definitely check out the zoo; I'm not a fan of zoos, but this one is lovely and well laid out in the middle of the city. the millennium park is great, and do consider a boat trip down the river to see the great architecture along the banks. Touristy, but has a nice view of the lake is Navy Pier.
Restaurants: where to begin!
Girl and the Goat
Leopold (good cocktails)
Trump tower has a nice cocktail lounge almost 8 years ago
Shopping on Michigan Ave. is a sport in itself. Definitely catch a Cubs game if they are at Wrigley that weekend. The Field Museum & the aquarium are both very cool and right next to each other. I can 2nd the Architecture tour as well - real interesting & fun to be out on the river.
David Burke's Prime and Cut are a couple of great steakhouses in the loop or Gibson's for an old school steakhouse that still holds up on quality (they invented the Gibson martini). Piece is a fun, laid back Sicilian style pizza restaurant with a good beer selection. And of course, you are going to want to try the real Chicago deep dish pizza while you are there - Giordano's, Lou Malnati's and Gino's East are the big three out there. almost 8 years ago
I second the boat tour if the boats are running in May - it's one of the best valued experiences in Chicago.
If you are a foodie, I also second Girl and the Goat but make your reservations now or you can walk in at 4:30 when they open almost any day of the week. On Michigan Avenue try the Purple Pig or a short ride away try MK, GT Fish & Oyster or NAHA. Little further away try AVEC or The Bristol. almost 8 years ago