I printed the cards with the prefixes on one card, root words on other cards. They match and see how many words they can construct. Make it a contest.
There's also a printable soccer game on www.esl-galaxy.com.
I've also used activities from this website:
I only use worksheets for pre-and post-testing. I do a whole group intro activity and then usually a practice task that requires partners or small group interaction.
If you don't care for any of these, just google-there are tons of websites w/ ideas! Good Luck.