During our stay in Barcelona in January we visited the Maritime Museum, which is situated at the end of Las Ramblas. It is housed in the former shipyard building, which is quite impressive to see. There are quite a few ships and boats to see and one of the highlights is a replica of a 60m long royal galley of the Admirals of the Juan de Austria from the Battle of Lepanto 7 October 1571.
Part of the museum was getting new exhibitions while we were there, there is a restaurant and a possibly to sit outside which we did enjoying the sunshine. According to their website it is closed on Mondays but we were there on a Monday, maybe that has changed so best to check there site. Kids are free which is also a bonus!
Since it is right across from the marina you can then go on a boat trip or just check out the boats. We wondered around the marina afterwards. This is just one of the many things to do and see with kids in Barcelona, we both found it quite interesting so worth a visit!