So many amazing beaches to choose from! If you are staying at a hotel nearby the most convenient would be going to Balboa Peninsula - rent surfboards, bikes, paddle boards, kayaks, etc! You can always text the bride or groom for recs!
Cruise the harbor in Newport Beach on an electric boat.
Take a walk around Balboa Island or stop into boutiques for some shopping. Recommend having a frozen banana and maybe taking the ferry across to Balboa Peninsula.
Crystal Cove Headlands Trail (or down to the beach), or Pacific Ridge Trailhead.
Shop around at Fashion Island