Are your piggies free range in there? If so you've got tons of options! Bendy bridges (Fiddlestix I think they're called!), cardboard boxes - get them free from your supermarket/homebase and cut holes in them, cat litter trays with hay in, chubes (Wilko's - £1.50), rabbit/ferret tubes (fabric ones), pigloos/hidey dens, foot stools to hide under! There's probably more that I've not thought of!
Piggies like the simple things; safe things to hide in, chew up, run through, destroy...