We've decided to try a video transcribing and subtitling activity as Leigh Thelmadatter suggested (
https://outreach.wikimedia.org/wiki/Education/Newsletter/April_2015/New_to_Wikipedia:_A_personal_perspective). We are using the Amara.org platform. I put up a list of videos for subtitling or translating from English; students may volunteer to pick a video or suggest their own (which nobody did as of yet) and transcribe / translate it. They get 30 points for the first minute and 10 for every additional minute they complete.
What I did not figure out yet is how to motivate students to do Peer Review. I offered 10 points for reviewing every minute, but students mostly ignored it and did not participate.
Thank you for all your suggestions. I plan to eventually extend this activity to Wikipedia articles too.