Upvoted, I think this is by far the best suggestion. Find a local charity or non-profit and help them get online or update their site, and add an easy way for people to donate online. Re-imagining a popular site to show off your skills seems like a vapid decision, much better to do something more meaningful. The charity benefits either way, and you've made some contacts which may help you in the future.

Some organisations may be reluctant to accept because they've tried this before and the developer moved on to other things. The charity can't be left with no clue how to update the website, so make sure you have a plan to hand things over to them. It can be something as simple as styling a Tumblr page mapped to their own domain.