Here is a 10-day, 9-night route that would be pretty cool, with campsite highlighted in bold: Discovery Island to D'arcy Island, to Roche Harbor for customs clearance, to Posey Island (very cool site, but only 2 campsites; reservations a must), to Jones Island (nicest campsites are on the southwest, second nicest in the south central), a brief lunch stop on beautiful Yellow Island, to Blind Island (another very cool site), to Turn Island, to Griffin Bay State Park, to San Juan County Park (this is the main leg for Orcas), back to Discovery Island.
The stretch from Posey-Jones-Yellow-Blind is the most "San Juans" part of the San Juan Islands, and the stretches on the west side of San Juan Island are very good for wildlife viewing. Jones has good day hiking, and there is some hiking on Turn, too. Posey and Blind are both tiny islands that give you an adventurous, castaway sort of feeling. Yellow Island is owned by the Nature Conservancy and is inhabited only by a caretaker. There is no overnight camping, but it's wonderful to wander around an undeveloped San Juan island.
Other than Posey, no need for reservations. Water available at Roche Harbor, Jones, and San Juan County Park. Fees required at all locations. Fees can be paid with cash on-site. Consult websites for amounts. Discovery is a BC Provincial Park, D'Arcy is a Canada National Park, San Juan County Park is obviously a county park, and the rest are all Washington State Parks.
Alex