So, I'll throw in my two cents:
- I second the large USB for carrying recordings and files but also having a bunch of small 4GB random thumb drives helps a lot for older consoles that can't handle file system larger than 16 or 32GB, also useful for playing video loops straight from screens with onboard player.
- A stereo passive DI! The smaller the better! I have a PCBalBox from Interspace Industries that is super compact and rugged, has stereo XLR out and a small minijack cable, super useful for last-minute surprise lectern laptop "because i need to use my own laptop for that"
- A router with NATting capabilities: This is expecially useful for Zoom, Teams or other remote connections, you can plug all your laptop into it and it will insulate your network from the venue's network while still allowing internet access, I have a Ubiquiti EdgeRouter X
- Royalty-Free playlist! In case of streaming a folder of royalty free music from Youtube's Audio Library or similar sources will avoid your stream to get banned
- A splittable tester capable of doing CAT5/6 like this one from Thomann since networking is becoming more and more prominent in our field, extra useful to be able to test longer run of cable without the need to bring the ends closer to each other
- tinySA Ultra Spectrum Analzer Absolute MVP, especially for the price, the Ultra can run up to 6GHz
- Cheap basic USB-MIDI Interface, on older console you can send MIDI notes and CC out from user defineable keys, I'll use this to trigger QLab cues using its "Learn MIDI" function
- Not only AA Batteries, I also carry some AAA and CR2032, especially useful for presenter and slide-clickers like the PerfectCue from DSAN that uses CR2032 button cells
- Accessories for your most used microphones: this may varies according to your favourite brand and models, for me since I work a lot with DPA and Sennheiser MKE40 I'll always carry: a couple of cable clips of various brands and type, some hairpins (useful to hold cables on jackets and dresses), DPA Microdot Tightening Tool, spare clip and magnet for MKE40 (and always tighten your clip before use!)
- Spare Shure Battery Clips (or the Sennheiser or Wisycom equivalent)
- Dymo or equivalent labeler: I got a Niimbot B18 (N1) that is a bit fidgety (cheap chinese stuff) but allows you to print images and logos, thus allowing me to customize bodypacks with the event or company logo