Second the trike idea. My dad was always active, but he developed balance problems in his late 70s and couldn't ride a bike safely, He bought a fairly cheap tricycle (might have been the Walmart one somebody else mentioned), and rode it all over his neighborhood until his death five years later.
It was too short for me, but I borrowed it several times just for fun and it wasn't horrible. Under the same circumstances, I'd probably buy a more expensive one, but I love to ride and he just wanted to get around.