I wouldn't worry too much about Holy Week being too crowded, depending where you are I guess... I heard Seville was crazy so we went to Cordoba.
My son (13) and I had 5 days before joining his soccer team in Madrid (where we went also to San Sebastian, and daytripped Toledo). He really wanted to go to Gibraltar to see the monkeys (I could definitely have skipped it, but he realllllly wanted to go and realllly enjoyed it, so I was good with the extra travel time).
We arrived Madrid and took a train the next day to Cordoba where we spent 2 nights (so basically 1.5 days). I booked a tour the first night through Arten Cordoba for the Holy Week processions - I thought he was going to whine and hate it. But he really loved it so much, we booked the tour for the next night too. Really I think he loved collecting the wax from the nazarenos, once he was brave enough to go up and say "Cera, por favor". We also did a segway tour which he loved (I was scared to death, but that's another story and more my issue!).
It was pretty crowded at night, but we were out with our guide right in it, so it was fine! I didn't find things really that crowded during the day. We were there Wed, Thurs (left on Good Friday for Gibraltar).
If you want to do Barcelona, another poster on here spent some Holy Week time in Zaragoza (Thursday to be exact), and really loved it, which is near to Barcelona. You could do Barcelona, Zaragoza, Madrid? We really like Madrid too. I would just maybe stay out of Seville (although i it may not be as bad as they say, I don't know). Cordoba only 1.5 hours train from Madrid too....
Also, it was chilly. Even in Gibraltar, it was nice, but I wouldn't call it warm. (This was last week of March though, not sure when Easter falls this year).
Overall I was really pleased that we got to spend some Holy Week time there.
Kim