Lots of advent calendars available online. Some of the better ones include:
NorthPole.co.uk have one with puzzles, jokes etc. You can open the ‘doors’ before the correct date (if you are the sort of person who likes to do that).
Woodland’s Junior School have an educational advent calendar, with facts about other countries. I’m assuming – it’s India today, but you can’t peek ahead.
Harpold.com provides a Christmas related item every day – today it is a poem. No peeking here either.
Billy Bear provides two advent calendars – one Special Advent Calendar, with no peeking, which provides free clipart, games, and programmes, and a Wallpaper Advent Calendar, with downloadable wallpaper.
BBC Radio 3 has Bach Advent Calendar with a quiz and music clips.
There are loads more.