Some great ideas here and no harm in having a 'big bang' type of event to generate interest. However do take some time to plan the adoption journey afterwards and on an ongoing basis.
You need to almost be continually supporting users as they learn to use Office 365. So using the likes of Yammer as a support network, lunch and learn sessions, workshops/showcase cafe's, video guides and much more all help to prolong the adoption journey and help people especially super users. One risk with such a fair to raise excitement is that it raises expectations and people are looking for something that solves all their business problems. And it might disappoint!