We will be in Athens 2 half days and one whole day before a cruise. My husband has never been and has trouble walking long distances. To get the best taste of Athens in this short time, should we hire a driver?

A lot depends on where you are staying. If the hotel is central there will be plenty to see without going far, & you would get the feel of Athens much more if you can walk some of it. What is too far for him? Are hills a problem (the Acropolis)?
Let us know your location/hotel & distances/time of comfortable walking.

Also consider a tour. CHAT gets good reviews here.

We have done CHAT and were very happy with them. Your hotel will have a pamphlet at the desk and can help you make arrangements. Easy and simple.

Have a great trip, if want personalized service at a reasonable price check out "George the Greek Taxi Drive" he/help all speak English and are great, we have used them several times, once to take us to a family wedding in a Limo. lots of fun. and they are very accomodating.
Good luck.