Toying with the idea of taking the family there in the Easter holidays for a short break, finances permitting. Just wondering what would be the better trip with two kids (13 and 10) at that time of year? Any advice greatly appreciated.
Depending on time, try and do both.
In Bilbao, its quite nice just walking along the river. The Guggenheim is an incredible and unique piece of architecture, but couldn't tell you whether its actually worth going inside. I seem to remember there was a kids park just next to it too. Some fantastic pintxos (tapas) available everywhere, but Plaza Barria was fantastic.
I was in San Sebastian 2 weeks ago. It's probably aimed more at couples than families, but I think the kids would have enough to do. Get the funicular up Monte Igueldo. There seemed to be a small theme park at the top which in truth looked a bit shit, but it might pass some time if its open (it was shut when I was there). The main reason to go up is the view over the city and the bay. The walk around the bay takes about 40 mins and is worthwhile. It doesn't feel very Spanish - almost English (but decent). The Old Town is essentially just hundreds of bars where there are loads and loads of pintxos (tapas) on the bar, and they give you a plate and you fill your boots. There is normally an additional menu though and in my opinion, the better stuff tends to be on those. I have a couple of specific recommendations if you end up going.
In the whole region you are hoping for good weather rather than expecting it, but people were swimming in the sea in January! The water was very clean but I didn't fancy it myself at that time of year.
If you're going to hire a car, you could drive up to Gaztelugatxe (filming location for Game of Thrones), or down to Vitoria Gasteiz which is another good city.
If you or the kids want to go to a football game then Real Sociedad (San Sebastian), Eibar and Alaves (Vitoria Gasteiz) are all at home for the Easter weekend. You won't get exact kick off times until about 4 weeks before though.
With Spain being quite religious, I'm not sure what happens if you are there for Easter. I imagine there are some small scale festivals which might be impressive, but also things might be shut. Worth checking before you go.