I'm a newbie to Geocaching, but have a lot of experience in field work of various types. When I do any kind of writing in the field, I use an "indelible pencil" which my dad introduced me to 50 years ago. Amazon has a bunch of them...local office supply outlets should, too. To me, the are much superior to most pens. The pencil writing/drawing does not run when wet or fade with time. I like black plus the multi-color set from Amazon, but just plain old black is fine. I have been doing field journals with these pencils for 50 years. They don't erase...you use a strike-through for mistakes...sometimes these are very informative on their own. (Regular pencil is good, too...still superior to pens IMHO.)
I also carry three different types of magnifiers. One is a big "Sherlock Holmes" type that is about 2X. The other is a fold-up at 5X and the third is a small 10X. I find that examining the areas around a cache (or fossil find, mineral find, etc) can be very interesting.
All data/observations go into a green cloth covered, hardback, lined "journals" from an office supply...I prefer the size that is about 10X15 cm.
I have learned to journal as much as I can about any outing...these journals are wonderful memories and useful data as years go by. I even record meteorological data from my hand held "weather station."
Just a suggestion...makes Geocaching a real scientific adventure for those who are into that.
Peace, Mike