Steve, summer before last my daughter and I did the following on a Saturday:
Left her apartment in Cambridge, MA in the am, later than I wanted to. Took the most direct route to Wells, ME. Ate very late breakfast at the diner across the road, where we had to wait to get a seat. Did Lighthouse Depot. Got back on the highway and went directly to Portland. Toured the Portland Observatory. Headed back south to Portland Breakwater. Lost a bit of time before I figured out the way I thought not to go was the way to go. Saw that one and headed south again to Spring Point Ledge which is real close. Then we continued south to Portland Head where you also can see Ram Island Ledge. Probably could have seen more lights too if we hadn't left the binoculars in the car way up the hill. South again to Cape Elizabeth Twin Lights. Looked for the other road south of the lights where you can see the active light from below. Took the wrong road and then found the correct one. Wasn't such a great view after all. South one more time to Goat Island. That took some time. Getting kind of dark now. We'd skipped Wood Island due to our later than I wanted start. Ate dinner in Kennebunkport, returned to highway and returned to Cambridge.
The previous year on a late spring Sunday she and I did the following:
Left Cambridge and headed north towards Newburyport. Stopped to eat at some diner. Must have been another late start as I remember it was a tuna sandwich. Saw the Newburyport Harbor Range Lights. Seems we got a bit messed up getting off the highway to get to town but we made it ok. Drove out to the Newburyport Harbor (Plum Island) Lighthouse. This took a while but not bad. Leaned on a fence and broke it. Well maybe it was already broken. Returned to the highway and went north to Portsmouth. Stopped at the nursing home to see Connie Small who wasn't in. Drove out to the lighthouse at Ft. Constitution. You can see Whaleback here too and I "think" I saw Isles of Shoals way out in the distance. The Coasties are friendly here. Headed north again to Nubble. Think we got on 1-A for a while. Don't do that unless you want scenic and very slow. Did Nubble and had a bite to eat in York Beach. I remember it was 4pm then. Returned via the quick route to Cambridge.
Another summer weekend daytrip we made was to head south from Cambridge to Fall River, MA to see Borden Flats Lighthouse. We got lost in town but eventually found it. Got a ride out close to the lighthouse with the harbormaster. Went to New Bedford where we saw the three lighthouses and two lightships. Ate lunch in town and took a mini walking tour. Went east to Ned Point Lighthouse and east again to see Bird Island. Can't get too close a view of this one. Returned to Boston for dinner.
Now these wouldn't be your routes but I just posted it to give you an idea of time. In general I've found that you can usually see more that you think you can, unless you're riding with my daughter who turns right when you tell her to turn left!