Monopod
Great for mixing up the angles and getting to those hard to reach places.
Colour Filters
A brilliant way to make landscape photography more interesting and a bit of fun to use. You can use in front of the lens of flash, a great technique to try when shooting double exposures.
Camera
I got this second hand Sony camera off ebay for £19.80. The viewfinder is broken but unlike my Sony A7RII I can throw this camera around and experiment without having to worry about breaking it.
Remote Shutter
Nowadays these come in different forms. You can get manual/digital shutters and even apps. If you’re shooting self portraits this is a great way to have more control in when and how many images you shoot. And if you’re into moving image, remote shutters provide another option to create stop motion or time lapses without having to keep your finger on the shutter.
Mirror
If you enjoy shooting portraits in or out the studio a mirror can be a useful tool and make the viewer look twice. If your in a creative rut a mirror can provide a great way to start approaching your images differently.