Short answer: not much. Where you’re going and for how long and with what objective will of course dictate certain additional stuff like food or a particular tool.
My bare minimum:
- iPhone - to look up stuff online; to take one geotagged pic with the Photo or iNat app; to view a map, aerial images, or drawings of the site; for viewing PDF files of site drawings/maps, survey protocols, etc…
- camera - nothing fancy, just a Canon Powershot (I’m on my 4th one!). It fits easily in a pocket, takes great macros, and records decent video/audio. My videos are often just to get audio. Later, upon downloading, I use VLC to extract an MP3 for uploading to Soundcloud.
- small ruler - again, nothing fancy, just a 6-inch clear plastic ruler with black numbers printed on it, like from a kid’s math set. I add yellow electric tape to the back so the numbers show up better (example: https://inaturalist.ca/observations/2842846).
- plastic grocery bag for cleaning up litter. I like my wildlands clean :-)
Sort of with me, but tends to stay in the car:
- GPS if I need to be collecting UTM coordinates instead of just lat/long
- core field guides and texts
- binos
- small containers or baggies for collecting specimens
- 1st Aid kit
- bug repellent, though I almost never use it. Things have to be pretty bad before I’ll slop that stuff on my skin.
- flagging tape
Particular clothing:
- hat
- light-colored pants (makes ticks stand out)
- safety “green tab” hiking boots