Here is a fun 20 - 30 minute activity. This PowerPoint has 6 slides with numbers 1-60. Have students think of a number 1-60. Proceed through the 6 slides and have a student tell you which slides their number can be found. Afterwards, you will be able to determine the number they selected! The method is provided in the preview.
See if any of your students can figure out the how this is done.