Hey there and welcome to the forum, Junior is very cute
His new cage is fantastic, lots of space to hop around and flop.
I would definitely recommend a larger litter box, a nice big cat one, or an under bed storage container work great for rabbit litter boxes. They like to eat hay while they poop so with a bigger box you can sometimes just put hay in at one end and then the litter in the rest of it.
As for toys, there are lots of options. Paper towel and toilet paper tubes, stuffed with hay, cut into a ringlet and dangled from the roof. Cardboard boxes (tape removed) with two holes cut in it. Junior will have fun making the holes to their required dimensions
Cat tunnels as mentioned, plastic stacking cups (for babies), my pair like to unstack them and throw them around. You can also hide treats between the different layers. Plastic baby keys (Luna favourite), wooden barrel roller rabbit toy, wooden bendy bridge/tunnel, sisal carrot. Paper bag stuff with hay.
In terms of flooring, coroplast can certainly be too slippery. Fleece blankets are a great safe option for bedding, other blankets and towels have long threads which can be a danger of entanglement or be dangerous when ingested. The only trouble you will have with the fleece, is that it too will slide around a lot on the coroplast. My cage has two levels and on the top level I hold the fleece down at the edges with clips, on the bottom level where I can’t do that I anchor the fleece under heavier items (the litter box on one side and a plastic hidey box at the other end). The fleece still gets moved around sometimes but it helps for their traction.