You'll find no shortage of things to do if you visit the Portland Parks & Recreation website. Actually it's astounding. Take a jewelry making class, water aerobics, computer education, or tai chi. Activities are sorted into categories for: Family, Parent/Child, Preschool, Youth, High school/Adult, and Adult 55+.
There are also specific activities that cater to the disabled. If you can't find it here, it might just not exist!