Welcome to the forums, Whiskers
The first and best thing you can teach them is to come to their name. This will open doors for giving them a bit more free range and teaching them more tricks. You should aim to spend at least a couple of hours a day with them, and more if you can manage it.
The best thing to look for in treats is to find something healthy that they like, so you can give them lots of little bits of it. Yoghurt drops are ok, but they're not really very nutritional and shouldn't be given in any quantity. You could try things like rice krispies, which are quick for them to eat, they won't get full too quick and they're not too unhealthy for them.
As for what to teach them, there are endless possibilities. I've even read of people teaching their rats to roll over! I taught one of ours to hop up into a hanging tube to receive her treats, and both of our rats will climb onto me when I say it's home time, now. Experiment with what you can teach them... I'm sure you'll be surprised. Remember, rats are very intelligent, so keep it varied for them and you should have some very happy smart rats!
