Thanks all. I'll definitely record weather. I think the extremes (max and min temperatures) are probably the most important ones to note as well as the amount of rainfall.
For future reference: After some messing around with various blog services, journal programs and online garden journals, I settled on Google Calendar. I'll just write a description of my observations and actions for the day and keep it on there.
The most important thing, if you're bothering to do it on a computer, is a good search function so you can find what you wrote years later. Google Calendar has that and can also be edited by several people which is important for me because I am away for nine months of the year.
Had I been the only person keeping the journal, then I would have used RedNoteBook (http://rednotebook.sourceforge.net/
). It has a amazing search function and is simply very user-friendly.