How about the Hugh Lane Museum? It's in Parnel Sq. They are showing an exhibition of stained glass artwork. Heh, I see on their website they run a Sunday sketching session for children. That might be fun too :) It's open from 11am to 5pm and admission is free. There are also free Sunday classical music concerts for an hour starting at 12.
Take a look through the site. There are some great exhibitions on currently. I think we could well spend the day going through it all. So, any takers?