To make the activity more fun and to help students learn how random assignment balances out background variables, you might have students randomly assign playing cards to condition, using the easy, entertaining, and educational exercise devised by Craig Enders, Jean-Philippe Laurenceau, and Rick Stuetzle. In the exercise, students act like the playing cards are participants and assign them to two groups. Students can then see that random assignment usually balances out individual differences. For example, there should not be many more spades in one "group" than in the other. (Note that there are a variety of background variables you could have students record: color, suit, face cards, total points).
The demonstration can be extended to look at the effects of sample size (e.g., 4 cards will provide less equivalent groups than 16 cards). It can also be used to demonstrate the problem of violating independence if you (a) use a new, unshuffled deck and (b) have students assign the first half of the deck to one group and the other half to the second group.
In addition to--or instead of--the playing card activity, you could have students go through the following online tutorial.Source of playing card activity:
Enders, C. K., Laurenceau, J. P., Stuetzle, R. (2006). Teaching random assignment: A classroom demonstration using a deck of playing cards. Teaching of
Psychology, 33, 239-242.