I always make dh take a couple of weeks off in the middle of the holidays so we can either go away or have a few days out as a family and of course, so that I get some respite. (I have a very two-year-old dd, a stroppy 7 year old ds2 and ds1 with ASD).
I find it helps to have some loose structure to the day, so we spend an hour doing educational (not necessarily in the academic sense) every day at roughly the same time, the boys have their nintendo time at the same time each day and snack and lunch time are also fairly routine. Personally, I hate being this restricted, but it really helps ds1 to cope.
We try to get out most days, even if its just to the local shop or the park (two minutes away). Being a mean sort of mum, I also insist on flinging them out in the garden at least once a day, whether they like or not - and they usually don't. 
I also get a stock of puzzle books, comics, mini lego sets and cheap dvds off ebay etc that can be pulled out on days where it all looks like its going to go pearshaped.
I feel your pain. I love to get ds1 back, once he's had time to relax a bit after breaking up from school, but all the same, the summer holidays are a very long time to be together 24/7.