What about a day at the Zoo with a picnic
If you want closer to home a picnic in either of the following (with a ball, kite etc)
Ardgillan
Newbridge house (has petting farm and playground for the kids)
Oldbridge House
If the weather is shitty then PJ’s in ballbriggan, brilliant play centre and they cafe is great, the food is really reasonable and yummy (check out the link on the right handside)