These are a bunch of simple and harmless (especially HR safe) pranks you can pull on your colleagues:

Wear a different name tag every day

Come to office with a different name tag stuck to your shirt every day. There are bound to be a few people who might trip up as to what your identity actually is and how to address you.

Fake moustaches

Just show up with a fake moustache one day. Don't mention it unless someone asks about it. Some people will do a solid double-take the first time you start speaking in a meeting.

Wear multiple name tags

In this one you wear multiple different name tags at the same time. And when someone asks you who you are, you just ask them to pick who they want you to be. And each name/character comes with their own different background story and what they do now and stuff like that.