In Dungeons and Dragons, and indeed: many other roleplaying games, (including painting blank walls for the warhammer / 40k!) there are whole suites of rooms that are described as "empty". This does not mean they are featureless. To the contrary, an "empty" room can consist of all kinds dressings and flavour.
One of the best resources I've found on the internet for this comes from the Hack and Slash blog (with a downloadable pdf here). This contains a wealth of information for role-playing games to make those empty rooms more interesting and entertaining. Add to that some traps, some treasure (pdf here), and the makings for a free wheeling game are already in ones hands.
Today, I wanted to give a list of graffiti for the Astulae setting that we have "in preparation". Many of these are generic enough to be transplanted to other settings easily enough. I hope you gain some inspiration out of them. Roll 1d10 if you must, otherwise select the ones you want!
1. Tag - a symbol or a combination of letters is scrawled on the surface at regular intervals, perhaps by disaffected youths, wild monsters staking their territory, or lost adventurers trying to keep track of where they've been.
2. Unreadable language - written back-to-front to what the players expect, or using characters that simply don't exist in their own language, this graffiti simply makes little sense.
3. Symbols of power - magical lettering and runes adorn the wall. Perhaps they're to be used in a ritual, or perhaps they summon a magical creature when read or traced by fingertip.
4. In Memorium - a stylised header lettering at the top announces those who have lost their lives, perhaps within the hallways of the dungeon being explored. A set of names follows the "In Memorium" header.
5. Arrows - several arrows point the direction to travel in. Or perhaps not. It is up to the adventurers to decide whether to follow them or not.
6. Warnings - "Beware the Red Telepath! She can take whatever form she wishes in your mind's eye!"; "Do not trust the Lady in Blue - she LIES!"; "Run away from all Crossers!"; "Always follow the wall to your left!"; "Do not pick up the ancient device!", etc.
7. Maps - an attempt to map out a portion of the Astulae (or dungeon) has been etched in to the wall. Several crosses appear on the map, perhaps denoting visited rooms, dangerous rooms, or rooms with vast treasure hordes?
8. Instructions - "Give way to Crossers"; "Stop at the intersection"; "No diving in to the pools"; "No walking across the grass", "Obey the rule of three"; etc.
9. Names and Labels - whether they are street names, house names, names on statues, or whatever else, the possibilities for such labels are literally infinite.
10. Audio announcement - "Attention please. The driver of the vehicle with registration XXX needs to report to reception immediately"; "No magic in the corridors"
Finally, whats with the image of the obliterator? Well, I've added some text to the base of the obliterator for a little flavour (slightly obscured in the image) that reads "Zoot Lives!".