In Belmont, there is a paved path known as the South Loop at Beaver Brook Reservation, that offers a self-guided urban tree walk and remembers massive oaks once located here. The trail has some steep grades, but is otherwise wheelchair accessible. With assistance of a physically fit companion, many wheelchair users can access this delightful path near Boston. There are several benches along the loop to stop and rest too.
I recommend parking in a 40 car lot on Waverly Oaks Road across from #520 to begin your excursion at Beaver Brook Reservation. You'll have to contend with a downgrade to get into the park (as shown in the first photo), but the paved path is 12 feet wide, which allows other people to pass by. The downgrade is 6-13% for 160 feet, followed by another at 10-18% for 60 feet - definitely significant, but doable with support if needed. A power wheelchair is unlikely to have an issue. You could also start out taking the roadside sidewalk and do a gradual descent into the park, then return the same way after exploring the park to avoid the steepest section. Just remember either way you will have to return uphill.
The park was well visited on a Thursday afternoon and likely is on any day of the week. No bathrooms here, so be forewarned.