Another expense, whilst it sounds silly, is chipping in for people's birthday/baby pressies/cakes etc, if your workplace is the kind of place that does things like that, or team afternoon teas etc. It's not much, but I'd probably usually end up spending about $20/month on that sort of thing.

Depending on your profession, there may also be professional membership fees. Since going on maternity leave, I'm maintaining my health professional registration (about $180/year), but have let membership of my professional body lapse (which is about $400/year), so when I return to work, that is an expense that I will need to take into consideration. Thankfully these are tax deductible.