As a gold, my last few SPG stays at Sheratons, Westins and a W included:
* Late (4 p.m.) checkout upon request
* Upgrades to club floors, larger corner rooms and, in one case, a "junior suite" that was actually one room but a large and very nice one at the end of a quiet hallway.
* Some misc. ammenities (which I didn't ask for) like fruit, bottled water and drink certificates for the bar.

I'll platinum soon, but the gold level has been a very nice experience already.