If your garden is more than a couple raised beds, a key part of your garden journal should be a map of your garden each year. If you're like me and don't account for pathways through your garden, keeping straight what is planted where is a necessity!
This map was just ideas and brainstorming.
A map will also help you rotate crops, so you don't plant the same thing in the same place several years running and exhaust the soil. My parents have a large garden, so they use a standard spiral notebook in which they always draw the layout of each year's garden, and write in the varieties of things that are difficult to tag- potatoes, tomatoes, corn, squash. They can also look at previous years' maps and see where they need to let the soil rest or plant cover crops the next year.
Mapping out a garden in advance is helpful if you have a small garden space but want to fit a lot in, like me. I actually took a measuring tape out to the garden and made a scale drawing of my space, and worked with the layout until I was able to fit everything in with the amount of space it needed to grow in.
My garden, and thus my journal, are a bit more modest than my parents'. I had a spare moleskin notebook hanging around so it got co-opted for garden use. It holds lists where I brainstormed what vegetables and varieties I wanted to grow, calculations for the cost of all the seeds I wanted to order, various drafts of garden layout, supplies I would need to find or buy, a rough planting schedule for the season, and notes/journaling as I keep track of the progress of my garden and starts.
I will also attempt to track the vegetable yield from my garden this year, mostly for curiosity's sake, to see how much money I saved by not buying my veggies at the store.
Some snippets from this year's journal:
"3/12/10. Potatoes bought today from Urban Farm Store, $3.95/lb. Russ. Banana, Yukon Gold, Yellow Finn, Red Norland."
"3/13/10. Planted out cabbage starts. Going to try and rig up a cover for cabb. w/old mesh curtain. Julie grew huge cabbages here last yr, so not overly worried @ pests though. 52 degrees F according to Wunderground, forcasted low of 40F. Supposed to be 56F and 61F next 2 days, so good news and warmer lows too."
"3/20. All replanting of peps + toms sprouted except Giant Marconis. Now I know- loose, light soil! Fast germination."