From Lake Dillon's marina (or the Frisco marina across the lake), you can rent sailboats, pontoon boats, paddle boards, fishing gear and almost anything to enjoy the lake. Website: https://www.townofdillon.com/marina
The Dillon Amphitheater hosts musicians weekly during the summer. More info: https://dillonamphitheater.com. You can find live music in Summit County, often free, almost any day of the week.
Paved bike paths run for miles between the mountain towns of Summit County, often next to the lake or streams. Rent a bike from one of the local shops! Or take a hike on any one of the many lovely trails.
Be sure to meet Isak Heartstone in Breckenridge while you are visiting. He's not to be missed! (Just ask after you arrive.)