Probably one of the easiest biking trails with numerous activities and amenities would be at Sunriver Resort south of Bend. Even if you are not vacationing at the many properties around Sunriver, you can just drive there, park at the bike shop and rent or pull your bikes off your car. Eat lunch at the village and play in the many parks. There is a free app in the app store called the Sunriver Navigator can help you navigate Sunriver roads and pathways, decide where to eat, and what to do.
Oregon Travel
Sunriver Family Bike Trail System
Take a break from biking and stop at the Oregon Observatory
Sunriver Bike Rental Shop