If you are a Brioni guy check out Morris and Sons, the Filene's on Michigan has Oxxford usually in weird patterns and colors, sometimes they do get something unique
I think about a block away there is Paul Stuart (sp?) that sometimes has some interesting things but I find sales people who aren't into the items they sell, there is George Greene which I don't think has any sales but interesting store;It is always fun trying to figure out who exactly his clientèle are and barneys in that block, Mr. Despos across from Barneys. There use to be Tender Buttons that was great for buttons around there. The Polo store is a few blocks from there and they might have some sales, if nothing else I like the restaurant they have there.
Brooks on Michigan might have some goodies, but terrible service, if you do buy and want something tailored take it to the Rookery and ask for Alex (tailor).
By the Rookery you have Syd Jerome's which sometimes has their 50% off sales on everything from Oxxford to Isaia but terrible tailors.
In that block is Albert Karoll (sp) at Richard Bennett but nothing "on sale" but great for working button holes.
Marshall Field's , now Macy's hasn't had anything interesting or unique since their change.
If it was me and I had the day, I would start at Truefitt and Hill and get a shave, then explore that little section of Oak Street and Michigan. Grab lunch at Sayat Nova , head down to the State area and check out the Rookery (note here, if you are single there are some really cute sales woman here<g>), Syd Jerome , State street shops then around dinner if you like Brioni or Luciano Barbera grab a taxi to Morris and Sons, after stopping at Manny's Deli and finish the day.