What hot cereals can I cook by just adding hot water?
If you try regular oats and like them, then you might try flaked/rolled barley, if you can find it. 

If you have refrigeration at work (or just bring in one-day's worth each morning), you could also add wheat germ (I can't imagine having a bowl-ful of it, but adding some to your oats would change them up.)

Wheetabix or shredded wheat can also be prepared with boiling water for a hot cereal.

Late recall - "Grape Nuts®" is another one that can be done hot, even if it normally isn't.