Well, as they’ve said: For certain, I would pick up - boots, gloves, knife, & lights.
The one thing I’ve had the most fun with lately is my camera (nothing too crazy, price wise), spent about $300 for the camera and housing, and does everything I need (take pictures/video underwater...).
If you’re diving without a charter, grabbing a few cheap spare items can save a botched dive. I’d pick up an extra tank o-ring (or 2), a spare mouthpiece (& zip ties), mask strap, batteries, etc... and bring along any tools you may need for quick service/repairs (screwdrivers, Hex/Allen, wrenches,etc). Typically with a good charter, they’ll have some of these things on hand, but you’ll be out of luck if you’re out there on your own.
It might seem a bit unnecessary with a wireless gauge, but having simple single analog tank gauge as a backup, can provide a bit of security should a battery die, the digital gauge leak/get wet, or just fail in general. Usually about 50-80 bucks, some can even mount without hoses(directly on the first stage), but you’ll need your buddy to tell you how much air is remaining.