WE have demanding boys too. We try to get them out of the house for as long as possible at least twice a day. Long walk, wrapped up in waterproofs if necessary, run in the park etc.. I think a structured day helps you too, esp when they're a nightmare: so you can at least tell yourself, right after sorting the washing, we'll have a snack, then the park, then lunch, then visit friends etc... Keeps your mind off worrying so much when you know you've got something to distract them with next... We just invent new activities like filing kitchen utensils somewhere new, or 'washing up' on the kitchen floor, or posting old magazines out the letterbox etc... Keeps them interested
Sometimes, in the middle of the tantrums, just opening the door and sitting on the front step helps calm ours down too.
Regular food is also crucial with our kids: they need a snack in between meals otherwise they are hellish.
And even though it seems like it might be harder work, it often isn't if you have another friend over. When your attention is focused entirely on them, they are at liberty to play up more; when other people are around, they get more used to entertaining themselves without adults. The more the merrier sometimes!