Blackrock Castle is a restored 16th-century castle that now, rather incongruously, hosts a small hands-on science centre, an inflatable planetarium and a pleasant courtyard cafe. Kids love it and the view from the tower is worth the jaunt. It's on the south bank of the River Lee, 5.5km east of the city centre; take bus 202 from Parnell Pl to Blackrock Pier, from where it's a five-minute walk.