There are so freaking many places to get ideas online, that, quite frankly, it's overwhelming. I have links upon links, but there's too much to get to know and actually use.
So here are some fantastic resources that I have found this afternoon, printed, and put into my beginning-of-the-year binder. They're all from the "vocabulary" section of WebEnglishTeacher. I'm sure that there are tons more great ones, but here is a handful to get you started.
Here is an interactive site that gives lessons and activities for the dictionary, thesaurus, and word parts. (Since my room doesn't have computers, I copied and pasted/retyped to make a worksheet/lesson plan.)
This site has a whole crapload of root words--and a special section just from Harry Potter. (I've been wondering who else was 'reading' the Latin spells and professor names!) Definitions and activities.
Obfuscations to help your students learn big, 'five-dollar' words. (Ex: neophyte's serendipity=beginner's luck)
More than one hundred "sponges" or quick activities. Perfect for Do-Nows! This one is good for all teachers of all grades (past primary, though).