Filters:
UV filters are essentially "screen protectors". I never use them. It's kind of like a screen protector for the iphone.
Polarized filters are neat because they reduce reflections
Neutral Density filters allow you to take time-lapse photos by reducing the brightness of a lens.
My personal favorite are macro filters. I've found the sweet spot to be +2. Larger magnifications are too distorted, but +2 is just right.
Other things to consider:
1. Extension tube for macro photography
2. viewfinder magnifier.
3. adapters for vintage MF lenses. I like this approach a lot because vintage MF lenses are really cheap and neat. and because this is digital, you have more leeway in pictures you take, while in the old film days you had to be more economical. Like, I use Olympus and the 50mm f2 Zuiko digital prime lens costs $400, but a vintage MF 50mm prime f1.8 only costs $50.