Driving this week from New Jersey to Bar Harbor. Would like to stop
on the way and stay overnight, because we think the trip is just too
long to do in one day.

Any suggestions for last minute stop? Hopefully
We can get a reservation.

Depending on where in NJ you are starting from it can be an 8 to 10 hour drive to Bar Harbor. Plot your route out on Google Maps or Mapquest and then choose a place either mid-way or at the approximate distance you are willing to travel on your first day. If you are diving on Interstates or main highways you should have no difficulty finding a room at one of the many motels you’ll find just off most exits on the highways.

Hint: There are websites for the major Interstates that will give you list of motels, restaurant, gas stations and other “facilities” at every exit along that particular Interstate.

From the Phila. area, I can get to Portland in a reasonable day of driving, and that's always my choice. Next day it's a 3½ hour drive to Bar Harbor.

Portsmouth, NH or Portland, ME depending on how far you want to drive. You should find hotels in either of those places that will be pretty close to the highway.

Although I agree with both posters, lodging availability may be low and prices high. For Plan B I would suggest putting Manchester NH or Concord NH as a possible stop. It looks like the detour might only add 1/2 hour of driving but be aware both I93 north and I95 north will have heavy traffic on Fridays and any day during commuting hours. You might actually have a quicker drive without traffic congestion if you head north from Hartford CT to Brattleboro VT then east to Concord. This is our choice for avoiding heavy traffic areas in MA. Be advised that from Brattleboro to almost Concord NH would be state highways, mostly 55 mph until you connect with I89 but beats crawling along major interstates during high traffic times.

I agree that 93 and 95 should be avoided on Friday afternoons and Saturday mornings but other times during the day it is not usually that bad. I was look at what Google maps says for when you are traveling. Going up through Vermont would add a considerable amount of time to your trip. I would try to avoid high traffic times and take the interstates. Bar Harbor is way up north. If 95 is bad in MA you can always go up route 3 in Burlington and then cut across 101 over to the coast in NH. (Manchester area could be a good place to overnight if you choose to go this way).