OUTSIDE, OUTSIDE, OUTSIDE! LoL... That was my life saver last summer. I had a 5 yr old boy in care that was.... difficult. Nothing was ever good enough, fun enough, etc. He fought all the time with my 4 yr old daughter. Luckily I also have older boys that he worshipped, so he did spend some time driving them crazy.
I also agree that during quiet times, sometimes the computers and electronics are a blessing. I used that time as a reward. If they behaved that morning, they would "earn" it. Any fighting with each other or other issues, it was removed as an available activity.
Just a caution; don't bend over backwards to "entertain" them. They are old enough to play independently for a period of time. I know that you are just searching for ways to keep them out of each other's hair, but I just wanted to say that if you have an assortment of age-appropriate activities available, they are more than capable of choosing what they want to do and doing it!