Oh and a tip for half term. Get out of the house. That way they have no choice but to miss TV and then, if they want to watch something later you can give in without feeling bad about yourself. It will do you good too.
DS and I, even now, enjoy getting out to collect leaves and acorns which we stick to brown paper or wall paper to make a big autumn picture when we get home. No TV til it's finished but don't ask for a masterpiece - ten minutes of sticking with a four year old is a huge amount of time for them. Or we just kick up leaves on a nice walk, may go into town and have a coffee for me and a piece of cake for DS (and me if I am honest!)
If it rains it's welly time and we jump in puddles. Amazing how jumping in puddles as an adult can make you feel so much better and your child will love you for it! Honestly DS told me, couple of years ago, I was the best mum ever because I didn't mind being a twit and jumped in puddles in town and an old lady told me I was a great mum for doing it (which made my slightly grotty day feel so much better). Shows how important it was, I still remember both remarks...
If it rains get some cornflour,pour it in a big bowl, add water and make magic gloop. If you haven't tried it, it is great. It feels solid but turns to liquid. Kids find it fascinating. Put a big piece of plastic in the floor though obviously. I have also been known to get DS's toy trucks and a heap of normal flour and helped him make snow tracks on the plastic. After ten minutes he is so absorbed I can just sit in the corner having a coffee and watching him until my energy level comes back. Easiest way I know if keeping DS amused even now! Don't try to be too clever with doing things, the attention span isn't always there at that age so ten minutes of something, then a snack, then a walk, then TV or read a story as some down time, then a run around rather than something they need to really concentrate on may work better. And don't worry if your DC asks for TV after ten minutes. Tell them it can go on in another ten minutes but only for one programme. Then TV off until after lunch or whatever. Alternating between doing things and having TV isn't a bad parent thing. It's life! It teaches kids that they can watch their favourite TV programmes but they need to be selective. DS now is given the choice of one programme or another, never both.