pull things out of the laundry as you get it done, especially undergarments and socks. These (unless they have favorites) will hardly be missed. Also, whatever you are packing these items in, place it back in it's regular spot so DS won't notice that it's out.
Depending on where you live and what your weather is like, you could also do some school clothes shopping for the boys and instead of putting it in their closets, put it into the suitcases before they even see it. They can't miss what they don't know is there! I think Disney t-shirts are on sale right now on their website and PJ pals are as well.
I once surprised my older girls, then 6 and 8 (well, G-pa actually paid for the trip) with a trip to NYC to see a broadway show for their birthdays. I packed gradually after they fell asleep each night starting about 2 weeks before the actual trip. They had no clue as we told them we were going to Milwaukee to see family. When we turned off on the airport exit, DD8 curiously asked where we were going. They honestly had no clue until we were at the airport. And just try to imagine G-ma's surprise when she arrived at the Newark airport to pick up a business collegue of G-pa's only to see her granddaughters walk out the door! It was a great weekend!
Good Luck with your surprise!