This is a special small town, and long day trip or overnight. We recommend driving around the island and stopping at local shops, restaurants, and beaches. In the summer this is a vacation favorite so make sure you book you ferry ride ahead of time and plan to arrive 30 min early.
A day trip to Friday Harbor is fun. You don't need a ferry reservation, the car cost is minimal and walk-on is free. A beautiful marina with shops and restaurants with decks. If you have a car check out Roche harbor, the outdoor art museum, Lime Kiln park and American/British camps.
Head to the Lopez Ferry Landing and walk on the free inner island ferry for a day of shopping in what Lopez Islanders call the "big city of Friday Harbor." Friday Harbor is more typical of a coastal tourist trap but is a great place to visit during you San Juan Island adventure.