You can always sign up to be a Standby Angel. Especially during the weekends there are often times for which you will be able to receive a last minute request, but even during the week parents are sometimes looking for a Babysitting Angel at the last minute. So, don't have any plans and want to babysit? Register as a Standby Angel and earn some extra money!
Go to 'Calendar' → 'My status' → click 'Standby' and select until when you would like to be standby.