I rotate toys for the pups. Something as simple as a empty big plastic bottle is fun for them. Also have some Little Tikes outdoor type toys (cube, fort) that they can climb into and around. A tunnel is also fun to explore.
Cardboard forms for pouring concrete to make posts make great tunnels. You can get them at major hardware stores like Lowes and Home Depot. Cost is less than $10. You can also use plastic pipes that are used for draining ditches. They come in small to large sizes.
I went to the dollar store in the pet section and purchased some big parrot (bird) toys. They have secure attachments and can be used as pull toys on the side of an xpen. Made of rope and wooden blocks. I have a Little Tykes toy that has steps, a slide and pass through holes in the bottom. I have small fabric chutes and larger solid plastic chutes. I put a crate in with no door. Then I rotate out between stuffed toys, chew toys, balls, etc etc...