Every once in a while I come across some idea that I want to remember to add into my games on occasion. So I'm going to add just a tiny list here and maybe it'll grow.
- Graffiti. I wrote about this before, but it is worth keeping in mind. It signals that you're not the first one there. It may be useful (warnings about what is ahead, arrows pointing towards an exit) or useless (Thomas was here). It's likely to be mundane or even vulgar.
- Inscriptions. Same as graffiti, but higher in register. Kings pay for tombs and towers and bridges and there's no reason they won't try to get it known as their bridge or wouldn't leave a record of who built it, when, and why.
- Occupations. If we're playing in a pseudo-medieval setting, most characters ought to have a pretty specialized occupation they know. Maybe they didn't follow in mom or dad's footsteps, but they probably know how to make a pewter mug or plate. And only a pewterer would know that shit. 2nd Edition DMG Table 60: NPC professions is a gold mine for odd little artisan backgrounds. Obviously country farmers may have a broader set of skills, but city folk are likely a bit specialized.
- News travels by word of mouth. Travelers are a great source of information. In a world where news travels at the speed of merchants and horses or ships, it should not at all be uncommon for an innkeeper to ask about the news on the road. In the same way the PCs should be able to ask about the town. Wizards with sending spells are hugely valuable to any large government or group.
- Wandering monsters. There's a few reasons to use random encounters (somewhat synonymous with wandering monsters), and one is to keep the action going. Sure it may seem like you're slowing things down with another combat, but even if the PCs are hiding in a dungeon room, the locked doors may draw the attention of a wanderer. And if its the right kind, one group of goblins can bring all hell down (or at least the rest of the goblins) on the PCs. Just make them reasonable: goblins patrol their encampment or just get drunk and need a piss (or a fuck?).