Pig stimulation is a topic I am often teased about when neighbouring friends visit and take the side of husband yes I am laughed at but I have content, confident stress-free pigs as a result!
My girls when outside have footballs, they have an old crate to which they can scratch themselves on, they have tyres which they can throw around, a wallow and me when I visit them and they push each other out of the way for the first one to be tickled and talked to (steel toe capped boots are a good investment by the way).
When inside, I have a radio on for the girls, and when the piglets are born I have soft toys suspended from the ceiling so they are kept clean, and once a week a different big toy is introduced just to keep them interested. As pigs are very inquisitive indeed it is important to keep pigs stimulated even if it means sitting with them for a while and talking to them not a good idea to do it whilst one of them is lying on your knees and falls asleep resulting in very numbs legs and pins and needles!!
Lots of human interaction is also a good pig stimulation this is the best tonic as this puts them in good stead for their new homes whereby they are very amiable. I have had a lot of comments about how my weaners who have gone to new homes have been so well behaved, loved their tummies tickled fall asleep on their new owners knees, like to play and are very demanding for attention!