head out to Phoenix park - bring a picnic - ring [broken link removed] and book bikes. They have a trailer for 2 if the kids can't manage bikes themselves. Head off for a few hours - monument, papal cross, find & chase the deer, look for squirrels, head up to ashtown castle playground, picnic somewhere in the park. total cost would be about 20 euro for the bike hire, rest is free.
ps . luas only 5 mins walk away so that would be an added treat - luas all way to the point and wander down to have a look at boats etc.