Since you were interested in ideas from teens as well, i thought i'd put my .02 in.
I'm 18, and for the last 2 years, I've "gone old school" and completely given up meat for lent. I know it would be much more difficult in a family that eats meals together regularly, but with my family's busy schedule, most nights its fend for yourself, so I didn't have to worry about inconveniencing everyone. I was careful about it, making sure to eat more peanut butter and fish (yuck!) and actually remembered to take my vitamins. It had a side affect too, because each week when we would go grocery shopping, if i wanted to get, say, some extras, like canned veggie soup or veggie burgers (a penance in their own right), i would end up trading a "snack food" like oreos or pudding for the veggie extra so i didn't affect the grocery bill significantly.
I also would make sure to pray the rosary every day (which eventually got me into the habit and I continued that past lent)
Although it doesn't apply to your kids, I want to throw the Facebook/myspace thing out there as well, for any others interested. I didn't last year, but about 75% of my friends (from my small all girls catholic high school) did. Wow was it quiet those 40 days!
giving up soda is always a popular choice, as is sweets. lol, I actually have a harder time giving up the sweets then I do meat!
Hope this helps some :)