This website has a ton of suggestions for things you can do in the area!
The cliff walk in Newport is a beautiful 3.5 mile stretch open from sunrise to sunset.
There are several historic mansions that you can tour (ex. The Elms, The Breakers, Chateau-sur-Mer, etc.).
Open from 12pm to 6pm on the wedding day. If you need to kill some time before getting ready you should check this out! Also keep this in mind as traffic might be heavier getting to the venue!