Try not to spend too much time on one activity--no more than an hour in the den on computer games, an hour in the back yard reading, an hour in the kitchen for eating, an hour on the porch to do arts and crafts. By having to change location and activity, interest is maintained.
A well prepared babysitter, whether it is an older sibling or someone else, gives parents great peace of mind and I commend you for helping your sitter be prepared!