As noted, benefits to Plats are up to the individual hotel, beyond vague recommended upgrade to better room. However, among recent stays, some HIs and CPs provide other things:
CP Ottawa, Club level upgrade all the time, in the past a bottle of jelly beans with the hotel logo on it
HI Southfield Michigan, upgraded room and bag containing bottle of water, pack of Oreos, bag of chips
HI Kingscross, Sydney, upgrade to suite with great view of city and harbour, drink certs
HI Old Sydney, upgrade to top floor mini-suite also superb view of Circular Quay and the Opera House, drinks certs
HI Select Chantilley/Dulles, upgrade to ExecFloor (no Club but lounge with coffee/tea/water and granola bars/donuts, vouchers for continental breakfast (or credit towards full breakfast) and free drink in the restaurant with dinner
HI Warsaw, upgrade to ExecFloor (no Club lounge), vouchers for 20% off dinner at restaurant, drink certs