Maybe a flight on the sea plane from Loch Lomond? We did this for my DH's 70th and he loved it. 45 minute flight, absolutely stunning views. You could have a nice meal before or after at Cameron House or Duck Bay.
My husband has limited mobility so I dropped him off at the wee jetty and I parked nearby at Cameron House. The crew were very helpful and there was a ramp to access the seaplane. My husband's 88 year old aunt, who uses a wheelchair, also did this flight on her birthday.
Wishing your DH a very happy 80th birthday.