When you are an au pair, your job is based around the children. Being with them like 10 hours a day, 7 days a week can make you feel like you're becoming a bore and that you have nothing else do to, but as long as you're with the kids and keep yourself open, you can do nothing but fun!
Ask the kids for ideas of things that they enjoy.
Are they artsy? Are they athletic? Love reading? Math? Cooking? Find something based in one of their special areas.
Ask the guest parents if there are any limits to what you can do, if they don't want the kids to burn the house down baking a cake, then don't bake a cake.
Pick something special for each week that will keep them on their toes.
If your children are artsy, then here comes my example......
Go to Ikea, pick up some of those super cheap wooden picture frames that are sold in the 3 packs.
Maybe a couple, 3 for each kid, and yourself. This costs roughly 1€50 per, but I'm sure the host parents will help.
Pick up paint, sparkles, glue, feathers.
Anything that your kid can handle that isn't too difficult to clean up!
Bring the supplies home and find a time that is great for all the kids.
Set up a table, call them down and decorate!
When all is done, search through albums with them (at the discretion of the parents) and help them find pictures that work great!
Clean up the mess, but make sure they help. You aren't the slave, and it will teach them equality.