It can be expensive to keep children entertained on weekends or in the school holidays, especially when you have more than one child, but one of my favourite things is to take my girls to the local library. They each have their own card and can use the library machines to take out and return their books.
My two girls (nearly 7 & nearly 3) particularly love going to the library. Even at a young age when they can't read it's a great idea to introduce them to books as even the colours and pictures will help their development. It also helps children to learn to look after things. Local libraries also hold free story telling sessions and often have colouring sheets and crayons and other free activities.
If you do take any books out I would suggest that you have a specific place for keeping them at home. We have so many books that we could easily get them mixed up so library books have their own shelf. This saves on panic when they're due back!
Borrowing a variety of books will not only help improve their reading level but will bring hours of enjoyment and could inspire them to write their own stories. One word of warning though...if they really like a book you could be reading it over and over and over again! "We're going on a bear hunt...." (arghhhh!) Happy reading :)