I probably would not bother with side trips right now unless you are looking to burn fuel (on the ground or in the air). Pretty much everywhere to the east of interest still has 3' of compacted snow on the ground, and it is not feeling very spring like yet. Portland itself has some half decent restaurants and bars in town and around the old port. Not so much out by the airport or mall, but I guess that it typical for most places.
If you are looking to burn fuel, a trip to Mount Desert Island, Acadia & Bar Harbor is doable if you get good weather. Only a small part of the park is accessible by car now, but it is a nice section. Plan on a good 3.5hrs to Bar Harbor by car from Portland though. Obviously it would be a snap if you are flying, and I think Columbia Air Services at BHB has a courtesy vehicle.
There are plenty of places on the mid coast that could also be long day excursions by car, and would all be much less driving than Acadia. Rockland & Camden would be good targets, and as someone already mentioned, there is the Owls Head Transportation Museum at RKD. It is small, but they have a nice collection.