So far I agree with everything said so far. Mardens in Sanford Maine is a wonderful place to shop and not too far. Of all the Mardens, I like the Sanford Shop the best.
http://www.mardens.com/index.php/stores
There is also a wonderful little Quilt Store, Kathys, right in Sanford as well.
http://www.kathiesquiltshoppe.com/
Keepsake is a must - it is about 60 miles NW from Portsmouth It is what I would refer to as an overload in quilt shopping but lots of fun! Their annual tent sale is June 3rd & 4th
http://www.keepsakequilting.com/retail-store
That is an experience that can not be described if you are there early in June.
One other quilt shop which is a bit away from Portsmouth but if time permits is Quilted Threads in Henniker NH - aprox 70 mi west(about 1.5 hour away) but it is my favorite place to shop (also the closest to me-lol).
http://quiltedthreads.com/
NE Quilt Museum in Lowell Ma is also a worthy stop - also not a difficult drive(a straight shot down 95 to 495).
I hope you have a WONDERFUL visit!