To avoid the cruise crowds at major sites (especially Trafalgar, Emeral Pool, and TiTou Gorge), try to be there before 11:00am or after 4:00pm. Otherwise you'll be overrun. But we had Ti-Tou completely to ourselves for 2 straight afternoons, arriving at 4:00 or 4:15. Harder to access sites such as Middleham Fallls, Boiling Lake, Freshwater and Boeri Lakes, and Syndicate Falls are less accessed by the cruising crowd so these are good choices for a mid-day activity.
In the middle of the day, relax, go to the beach, eat a good lunch, or do a little shopping. Or go to further afield sites.
If you're a non-cruise visitor to Dominica, you will be happy you followed this advice.