I sometimes get my 2 nephews (12 and 7) and have the same problem. Here are some of the things we have done, don’t know if they will help you or not:
Had a Treasure Hunt in the backyard with various trinkets from the dollar store and similar places.
The 12 year old needed to work on his Boy Scout “mentoring skills” so he helped the 7 year old learn to swim under water while I supervised
Bought a bunch of cheap capes (Target had them for $1 each) bandanas and toy swords and played Pirates out in the woods behind my mom’s house
Bought plastic golf balls and some cheap clubs at Play It Again Sports and taught them to chip and putt in the backyard
Made them plan a meal for their parents, took them to the store and made them pick out all the ingredients then they had to cook it…it wasn’t that expensive, just did spaghetti, salad and bread.
Hope that helps, Good Luck!!