Lots of things to see....there are lots of freebies--but per my coach...by Saturday that may be thinned down a bit.
They have lots of vendors--they have a HUGE display of official merchandise--though this is available in some of the hotels as well. There are running merchandise vendors all over the place. Stuff to cure what ails you, garments, shoes---just fun to look at it even if you don't plan on buying anything.
Allow 30min-1hr to pick up your race packet and shirt (you signed in upstairs and then went downstairs to get your bag and shirt last year). Allow 1-2 hours to walk around the expo. So if you get there at noon--plan until about 3 or 4pm at Expo.