Shuttles that I've ridden so far,
Stenys (800 S 2nd St, Milwaukee) - Just need to buy food or beverages. Then get your arm stamped. First shuttle leaves an hour before the game, then like every 20 to 30 minutes. Then return shuttles begin the top of the 8th and 15 minutes after game until they get everyone they stamped.
Kelly's (5218 W Bluemound Rd, Milwaukee) - Same as above. Minus the arm stamp. They did/do offer parking for $20 in their lot, but you did get a $20 voucher to the establishment if you did so. Not sure if they still give the voucher. Don't remember the frequency of their shuttle, though they do run many trips
Who's on Third (1007 N Doctor M.L.K. Jr Dr, Milwaukee) Same, buy a beverage and ask for a wristband for the brewers shuttle. Don't remember the frequency of their shuttles to/from the stadium
All were good experiences (coming from a upper 20s guy)