I just finished an interesting idea for a level I had: LittleBigSudoku. Someone else probably thought of it too, but the idea as I implemented it was to make a grid so people could trigger doors to open by placing the correct number stickers in the correct square. To top it off, it uses a race so that your score is based on how fast you solve it.
If you are interested, come play and heart me. My game ID is FadeToOne. :D