A simple way to avoid extra stress is to prepare a tool kit for your guests which are given out on arrival. These are filled with everything your guest will need to partake in the 'voluntary games' such as 'pin the dummy on the baby' and 'Guess the weight, date & time' that you have set up. That way, you can focus on your hosting duties without being asked for a pen one hundred times over ;).
What to include in your Baby Shower Guest Tool Kits:
A pen / pencil
A postage label with string for adding to a wishing tree
A piece of card clothing with room to write their baby predictions on
A peg to hand their predictions on the line with
A straw for their milk bottle drink
A badge / sticky label for their name & relationship to Mum
A photo prop (baby themed of course!)
Game Instruction sheet (for all games being played on the day)
You could even include a map of the house detailing where the toilets are!