There are lots of things your boys can do at or near home. If they need a goal in these activities, maybe it can be to show their Daddy, who works so hard and loves them so much.
Bring them boxes - old storage boxes, big refrigerator boxes, any kind of box they can play with. Let them make the boxes into whatever they want.
Go on bug hunts. Give the boys pads of paper and a pencil, and have them draw any bug they see. Then post them on a wall as if your kitchen, hallway, etc., were a museum. Another week, have them do leaf hunts, or rock hunts, and they can draw those. Drawing "ability" is not necessary. Ask your husband to be the museum-goer so your "curators" can show their exhibits.
Take them to the library on reading-aloud days. Read to them aloud at home as well. Have your boys collaborate on a story they can tell to their Daddy (and if it's a rehash of a TV show or a book they've read, it's no big deal).
While you're at the library, find a CD of old American folk songs - oldies but goodies like "I've Been Working on the Railroad" - and let your boys learn them so they can sing them to Daddy.
Encourage your older one to read any books he thinks might be interesting, while you read in a different room to the other one. (I have a granddaughter who says reading chapter books is boring. I just sent her a good book with a hand-drawn "boredom not allowed" sign affixed to the front with scotch tape. I hope she'll laugh and read the book.)
You want to take advantage of the comparative openness of a summer schedule to get your children to imagine, to think, to try things on their own, as well as doing the traditional summer activities of swimming, park play, etc.