Groupon, a new daily e-mail blast that offers group discounts on “cool stuff to do in Chicago” when a certain number of people sign up for the day’s deal, is offering $50 gift certificates to the sushi restaurant Tsuki for $20 (a 60% discount). There’s no minimum purchase; the only restriction is that only one certificate can be used per visit. You have to join Groupon to get the deal (good until midnight Sunday), but it’s free, and the 35-person quota for this deal has already been met. Plus fugu (puffer fish) is now is season, and Tsuki chef Toyoji Hemmi is the only chef in town licensed to prepare it.