A gardening journal is a valuable tool in figuring out what works best in your own garden. It will be most insightful if kept over several consecutive seasons. Because figuring out what works best takes a lifetime to fine-tune, really. Even long-time gardeners experiment with new varieties, struggle with newly-introduced pests, or need to adapt to changing weather patterns. Tracking things like your garden layout, important dates, seed/plant performance, weed control, and expenses can help you max out your garden's bounty.
If you’re thinking that idea journals are for fully-formed, brilliant, world-changing ideas, stop right there. Idea journals are for all kinds of ideas, including those that need to be incubated for awhile. These journals can include all kinds of things, such as daily goals, achievements, observations, quotes, artwork, and ephemera. They're not so much about fully fledged ideas as they are about inspiration.