The humble pencil is simple genius! The more modern mechanical pencil has bumped the pen as my daily writing implement, consequently my diary is a lot tidier, being able to rub out changes rather than the scrawl of pen that usually results!
Use pens with refillable cartridges, not disposable ones. If you do still want a disposable pen, these biodegradable pens might do the trick.
Avoid using permanent markers where possible, try paint instead.
Avoid felt tip pens, try colouring pencils, crayons etc. Artist quality products have excellent colours and will last for years.