We like to break up long car jorneys with a stop over at a National Trust property (free as we're members). Just thought it would be nice to share tips over which ones are good to go to with kids as sometimes its hard to tell from the handbook. So;
Belton House, Lincs (off A1 near Grantham). Fantastic adventure playground, mini-train rides, formal gardens as well as the house. House has one room with old Victorian era toys that children can get hands on with. Also in the old stable block is a kids' craft room where there is normally some type of crafty activity to do.
Clumber Park, Notts (near Worksop). Large woodland/parkland and lake. Great for walking, cycling and picnics.
Waddesdon Manor, near Aylesbury. Good adventure playground, aviary with rare, tropical birds, gardens. We didn't go in the house.