I hear where you are coming from - it's hard work and I'm a freelancer so I'm trying to work at the same time too (at the moment I'm working and the boys are actually playing really well with the train track on the landing outside the office...).
It's all very well going out and about but you can't do it every day. Going out is hard work on your own too - I've got 6, 4, almost 3 yr old so it's not easy, it can get quite stressful can't it?
Must admit, I don't have the badgering for toys or anything and dh is willing to take time off which is great.
Tbh, you need to tackle that - it's out of order for him to leave it all up to you. Can't he see you are at the end of your rope? Ask him what he is doing with all his holiday. Take yourself off for a weekend, like he does, and let him spend some quality time with the children. I certainly would not put up with this (I know people in RL that struggle like you do and I don't know how they do it).
As for entertaining the kids - when staying in I agree with Nutty that just chucking them out in the garden is good. Even if it's wet - I just put wellies on mine the other day and they played out for ages. Paint brushes and water is a good one. Get them to wash their bikes or ride-on cars etc. Get them to wash your car!! I've also saved up loads of empty boxes, bottles etc so that one day we can just empty it all out on the kitchen floor and make junk models.
How old are your kids? Do they have playstation or anything? That will usually keep them quiet for a while!
Do you have any friends you could hook up with? Maybe there is someone that could have yours over to play for a morning and you could return the favour? IN my circle of friends we've done qutie a bit of that this holidays and it's worked really well.