One of my most useful items are a set DIY viewfinders made from black ABS sewer pipe fittings. When combined with a black matte board cutout glued to the front openings and calibrated for each matching lens's field-of-view, they exactly match the projected image on the 8x10 ground glass, when pressed to my eye socket.
I find that some of the most enjoyable LF photography time I spend is done before hauling out the camera, when I am just walking around looking at things. Much easier to use these guys (one for each lens I use) to do a quick ballpark assessment, then just keep walking.
If the composition is worthy of future consideration with the actual camera, I will often mark the spot on the ground with stones, or whatever else I can find, and make a note of the location. Then I can return later when the light, time, or weather are better suited. I have even used small spray-painted crosses if I think I might need to wait months.
Another useful item is my classic Brunton pocket transit (magnetic compass) from my long ago days working as a field geologist. I use this to record bearings of various subject matter if I feel the direction of light might be better during a different time or season of the year.
I have used such readings in the past together with various software applications (planetarium programs, Photographer's Ephemeris, etc.) to carefully narrow down dates and specific times when it might be better to return. Don't want to take a day off work and drive 200 miles in March when August would have been a better choice.
That was the case with this photograph, when, to get the glancing sunlight as I wanted it, I measured the orientation of the wall with the Brunton, then waited almost nine months to return on a specific day and at a specific hour of the morning.
And a third really useful item that I, and a lot of others, carry is a small battery-operated voice recorder for notes. The saved information density is much greater than with written notes. And I can use it in the dark when necessary. This is especially useful to me when, as described above, I am just walking around looking.