A lot of kids love obstacle courses. They’ll keep going on them for a while, sometimes adding new obstacles or changing the rules to make them more challenging. Best of all, you decide what items can be used to make obstacles, and it can be done indoors or out.
A simple 2×4 makes a great balance beam. Obviously you should use great caution in allowing any sort of raised balance beam; make sure it’s secure and age appropriate, but even just on the ground works for younger kids.
You don’t have to buy any special equipment; just take a look at what you already own. You can line up chairs to make a tunnel through the legs, for example, or have the kids dribble a ball to the next obstacle.