Toilet/kitchen roll holders are good, you can fill them with hay, that keeps mine occupied for a while. Cardboard boxes they love to hid in and chew too.
You can buy hanging down metal balls that you can fill with hay/veggies mine love this too.
Mine have got a couple of the plastic balls.They don't really play with them,Freya will sometimes pick one up in her teeth and throw it round,ussually when it is in her way.
They enjoy toilet roll holders filled with straw,keeps them entertained.Remember to cut the roll though so they don't get their head stuck in it.
my piggie have a log, a pigloo a log arch and this ball thing that granny piggy got them lol, its diff colours with holes in it has most of shape missing as in like 1/4 at all 4 'sides' lol they liek to thow it about along wiht each other :S
My boys have a little ball made out of twigs or something. They love picking it up and shaking it about or rolling it around the cage. They sometimes accidentally hit one another with it.
Gilbert is a little chewer as well, he has tons of hours of fun just chewing on toilet rolls on little pieces of newspaper I put in there.
I also got two little small wooden funnel like things they like to walk through or climb on.
Ludwig makes his own entertainment by jumping up and down the top layer of the cage. I have no idea how the hell he manages to jump up there. It's about a foot or more off of the ground @.@