Anything that makes a tunnel....example, cut off jean legs (with a hairband or piece of rolled cardboard inside to make them aware that the fabric contains a crawl hole)
Tacky as it may be, they LOVE cardboard boxes. Of all sizes. Trash & replace once a week or so. Pasta noodle box, lightbulb or beer bottle box, etc
They really really like those stretchy rubber balls with soft rubber spikes on them. It's like a gel-rubber & the flimsy flappy rubber spikes look a blowfish.... i get them at Walgreens
Hanging strips of fabric & dangling rope. Swoop them above/across to so they sit like a hammock.
Buy a bunch of cable zip ties, keep them in stock - and just HANG THINGS & attach climbing levels, anything with holes in it can be zip tied to cage & used as a perch. They like to do run-throughs of "complex" obstacle course type setups...makes them feel cool to zip up & around their cage when it has a stealthy sneaky vibe to it
Lightly wadded newspaper. They have a blast with deep layers of bedding, too. Especially if it contains a tupperware hut shoved in it.
For baby rats: anything SHINY and/or dangling is an absolute must. They go nuts on little rope obstacles too, a series of dangly bird toys etc to latch on to. And also for baby rats, they're obsessed with little oven mit pouches. Pouches are a big deal. They play and play and play with the dumbest stuff.
Plastic easter eggs, hide cheerios & seeds in them. Boiled shelled eggs, let them roll it around & figure out how to crack the puzzle.
Old purses are a big hit. Hang them.
Variety. Add/change a few things every few days & they'll act like they've been taken to the circus. Even just one crinkly cereal bag is a freakin joy ride.