Personally I LOVE the summer holidays but I do know where you are coming from. My DH works away alot for months in end and when my girls were younger, i remember thinking OMG - 6 weeks!!
Try and get all of you out at least once a day. Even if at worse its just a walk to the local shop for a pint of milk!
I do stuff like make a pic nic lunch (nowt fancy) and take the kids to the local park, the park in the next village, the park 5 miles away etc etc. Free and easy - as long as the weather is semi decent. Its actually better on a slightly cooler/duller day with 3 kids to watch I would imagine. Good parks get crammed full on hot sunny days and makes it hard to watch all the kids! Suggest going with other mums and kids to make it more pleasurable for yourself. The pic nic element for some reason makes this so much more than just another trip to the park from the kids POV! LOL.
If its really hot and you want to avoid the play park crowds, do pic nic in the garden with a paddling pool or even really old fashioned stuff like a blanket over the climbing frame/2 deck chairs etc and make a tent/den and suggest games, get the teddies out etc. Invite a friend over for your older child and hope they send an invite back meaning that another day you can just do something with the 2yos.
Pair up with another mum and suggest a day out somewhere. Zoo, beach, soft play etc.
Find out about local leisure centres, sometimes they do a weeks long course in mini football (1 hour every day) or swimming, trampoleneing etc. Check out local museums as they are free. Alot have special kids exhibitions and free kids activities on in the summer hoilidays. Save this for wet days. Libraries also sometimes have stuff on as do local shopping centres - they may have something going on. Start buying a local paper or checking noticeboards in your local town.
Start saving your yogurt pots, cereal boxes etc for wet days activities of junk modelling, perhaps plan some home baking with the kids etc.
Try not to put too much pressure on yourself. I rarely tell my kids what I have planned until the day before. Just in case I can't face doing it myself the next day when I wake up. Use the holidays to your advantage. Don't feel you HAVE to stick to rigid routines and bedtimes (If it suits you). Sometimes its easier to have a routine but its the holidays so you DONT have to worry about school etc. 1 late night here and there wont matter.
I try and let a certain amount of mess wash over me with and console myself that come September I can have a big springclean and clear out! Even so, we usually have one major clean up day in the holidays when I get fed up with stepping on Bratz dolls earrings and lego!
Good luck!