one of my favourite is a toilet roll stuffed with hay but leaving a slit in the roll all the way along. watch them enjoy chasing the cardboard tube around for ages. Paper bags filled with hay, the love the noise!
Tunnels, hay tunnels and carrot cottages - frankly anything they can eat!
Brown paper envelopes stuffed with hay and coriander, I bought 500 A5 size brown paper envelopes for £2.95 and they make a great piggy treat bag also hay cubes, carrot cottages, woven grass hidey tubes... and whenever we get an Amazon delivery (usually of piggy supplies!) I cut 2 doorways in the cardboard box sides then stuff it with hay and let the piggies play in it all day