We live in the Midcoast and try and camp every other weekend. We take a 2 week trip at the end of July in Maine's Public Reserved Lands, every year we pick a different region, that has been a trip.
Our favorite campgrounds for weekend camping are as follows
And not in any particular order.
http://www.meadowbrookme.com/ - has a pool, playground and is 10 minutes away from the ocean.
http://www.lakepemaquid.com/ - we probably hit this campground the most, it's only 20 mintues from our house so we can usually just pick up and go. We never have to reserve a spot, but it has the campground vibe. Has a beautiful lake, heated pool, and 4 hot tubs. It's also really close to Pemaquid beach and my most favorite Maine attraction; Pemaquid Point.
http://www.chewonkicampground.com/ - another favorite for us. Only 5 minutes from home lol They have a nice nature walk, the area is just beautiful, has an awesome salt water pool in addition to the wading pool.
http://www.thomaspointbeach.com/ - is really a beautiful park, has tons of space for kids to run free, awesome fields and a great playground that I still enjoy after all these years!
Have fun, we started exploring Maine more a few years ago when gas prices sky rocketed, we've hit a ton of campgrounds but the above 4 are ones we will continue to go back to summer after summer.