There are a lot of great accessories, but the ones I have are often pretty game specific. I debated between Campaign Coins (I use the Fate Tokens and the D20 Crits or Fails coins for Inspiration in my D&D 5E game) and Dwarven Forge. For pure geekery, I’m going to go with Dwarven Forge. They’re like building blocks (or Lego bricks) for GMs. If you use miniatures, they add another dimension that really makes an encounter or location memorable.
Of course, if you don’t have a table with a central inset surface, they can cause visibility issues. They’re also expensive. But, they’re awesome accessories. Most accessories are luxury items by nature. Back in the day, we didn’t use miniature, battlemaps, or heck, even character sheets!