Here's another good one; the soft side of one of those really sticky velcro squares. I found one on my desk from my internet install kit. Gives it a nice tactical feel. You can cut the big square into smaller ones and you have a few replacements.
I just did it to mine. I needed something to stick out more than scratches or some of the other good ideas on here. Just in case I'm doing something like looking at the map then have to jump over to the movement keys when I get attacked from behind maybe.
A "W" key is a "W" key. If you can't see it behind the velcro you're a gamer so you know where the key is anyways.
I have a black key-white letter keyboard, the ivory Logitech K360 to be exact. I also put clear packing tape down in squares, color coded my "R" red (for record. music production.) "G" green(ish) (for Google Drive) and "P" yellow-ish (for photography folder. landscape art for showing quick passer-bys my shots.) and put a second tape over the colored sharpie ink so it's see through.
As you can see I love using my hotkeys. I use them for everything. So I thought I'd put this one in about the velcro stickies for future readers.
There is now (and was before) a WASD "keypad" by Nostromo they used to sell in Best Buy, Gamestop, Electronics Botique back when games like C.O.D. first came out and all that OG stuff that started the online WASD thing. There's another one out now by Logitech called the "G13". They both have thumb joysticks/d-pad for movement or whatnot. You can find them both online. They have a more simple interface, custom keys and the G13 has pre-indented "WASD" numbered keys so you can find them faster.