Make your notes publicly available (within your organization or team).
Securing your notebook will be an easier option, but as you've asked for other approaches you might consider this one.
This will be varyingly convenient depending on how your workplace operates, but if you have something like a Sharepoint site or internal blogging service you can keep your future project notes there instead of on a notepad in your desk. Alternatively you can meet regularly with your boss and go through your ideas for new projects. As long as it's only accessible to the appropriate people in your company or workgroup you shouldn't have to worry about confidential information leaking as a result of this practice.
You can be more creative as well, making things like ranked-choice surveys for executives to vote on which projects they think should have the highest priority, etc.
In any case you are creating a contemporaneous record of your ideas in a manner which identifies them as yours-- update histories, your boss' notes, and so on put ideas "out there" but still identify those ideas as your own.
I, personally, would also include a note in my notebook outlining that I know what the snooping coworker has been up to and am not pleased about it, ideally with some kind of tamper-evident countermeasure (like a thread taped across the drawer which will be broken if the drawer is opened). A change in your practices will be obvious to the snoop anyways, so there's little reason not to be a pointed in making sure they know why those changes are happening.