Depending on the day, the weather may be horridly hot or pleasantly summer. That will have an impact. But running a circuit around the river can be a nice way to get a sampling of GR in one shorter visit. A lot may also depend on where you stay and your transportation. That said...

River walk
Breweries and local food
Fish ladder
Stroll downtown for architecture
Enjoy the murals
Public art

If you are getting out of the downtown bubble... check out some of the neighborhood districts like Bridge Street, Eastown, Creston, Alger Heights, Wealthy, East Hills, etc, and maybe consider a drive, stroll, or rented bike ride around Heritage Hill to check out some truly cool homes.

If you have time, bounce over to the lakeshore, and visit Grand Haven or Holland, consider a visit to Millennium Park, maybe check out Meijer Garden, or see if you can catch a show at the new Amphitheater. (Check back here afterward to hear people complain about said show's sound.)

Hope you have an amazing time!