go to Newgrange if you can manage it and are into history in any way. Its a fantastic experience and IMO a brillaint way of managing an ancient monument. One of the few places where Britain could take a tip from the Irish for places like stonehenge.
Well you can check out Stephens Green Shoping centre and park and send the lads off to Killmainham Jail (Its now a museum) Thats where they executed all the leaders of the 1916 rising and all that kinda stuff. the National History Museum is also fantasic and there are some good art gallerys. If you have a spare 1/2 hour or so the Natural History Museum is worth a look as its an old victorian style Museum full of stuffed animals. Its one of the few of this style of museum left in the world so is kinda a Museum of a museum IYSWIM.