Trainer: Hey. Hey. You know how when you stealth, you walk really slow?
Trainer: You can just walk faster while doing it, with no downsides.
Rogue: DEAR GOD THAT'S BRILLIANT! WHY DIDN'T I THINK OF THAT?!
Trainer: 2 gold plz, kthx
The one I'm really curious about is the Druid's cat form. For bear form and sea lion form, they have to do a complicated quest... but for cat form, all you have to do is give your trainer a few coins.