I have trained my 3 pigs now especially one of my pigs she is my little genius (and she loves doing it, she gets so excited!). She can do all sorts of things including picking up only the red ring when I set down 3 different colored rings. My other pigs are better at other things of course each pig is different in what tricks they might be good at and what they enjoy doing (some pigs might not like training so much! so respect your piggy).
The best treats are veggies, but cut up super small. You want something quick so as you say "good girl" or "good boy", click your clicker, or whatever means they completed the task correctly, it is instantaneous with no delay between the action and the treat. If they take forever to chew they may not associate it as well. It also makes them learn faster and you can do more training in one session as repetition and consistency is key so you could easily feed them their 1.5 cup daily veggie amount during just one session if you aren't careful haha.
Just some advice: You must continue training each and every day, and at first it helps to only do one specific trick per session with maybe 2 sessions per day max to let your piggy take it all in. Every pig is different but of course you should take it slowly and start really simple. After the first few tricks they will have learned how to learn more tricks if that makes sense so it will become easier to teach new ones. And of course make sure the pig is happy and comfortable during training. Start each new trick doing it exactly the same place, like a spot on a towel, and with no distractions. Once they know it really well you can try it with slight distraction like another person and maybe on a different surface. They know to do the trick exactly the way they learned it so if you only taught them on a towel with no distraction saying it a specific way they might have trouble if you say that command somewhere else. Just something to remember. Guinea pigs are super smart and it can be a great bonding experience training your piggy as long as you are patient and caring.