Probably can't go wrong with any steakhouse downtown. I've been to Morton's and The Capital Grille, both of which were very good and about what you would expect price-wise. You may actually have a more private table at Capital Grille. Also, it's kind of nice to celebrate and relax during dinner with some champagne or wine, so proposing during a walk around Millennium Park beforehand wouldn't be a bad idea. You can enjoy dinner that way. Congrats!