Perhaps you could get some help from someone. Are your dojo affiliated with some sort of organisation that could perhaps send a substitute sensei ? Maybe one or two of your sensei's old dojo-friends could help out ?
I have myself tried to put on a Kendo-demostration based on 3. 4. and 5. kyu's and it never really had that 'thing' that made it appealing to the spectators. Basically it is very hard to 'fake' proficiency, and therefore virutally impossible to explain what Aikido COULD become, when one is not yet there.
I think the best thing you could do is to be true to what you already know and to show what you've got. Do the techniques that you feel confident about and drop the fancy 'high fall' stuff if your'e not up to it. After all honesty is a very important principle in Aikido.
I had a talk with my sensei about a similar subject yesterday. My question was how much practice one should have to open a dojo, and he basically said, that as long as you don't pretend to be more than you are, and as long as you don't put yourself on a piedestal and claim that you 'know the way' then people who are honestly interested in the art will be ready to follow you on the path towards learning Aikido, but if you claim to be a competent instructor they might follow you for a while, but when they one day sees your shortcommings they wil be dissapointed and perhaps loose interst.
I understand that you want new students very badly - but I suggest you focus upon getting new student who actually WANT to learn Aikido instead of those 'I just want the quick four weeks black belt / killer machine course' kind of students. I think it is unfortunate to try to 'sell' aikido to potential students - as far as I know it is very rare that a demonstration is followed by a massive sign up of new students. It is far better to show what Aikido is to you, and perhaps you will mannage to tickle somebodys couriousity and this will eventually grow into a desire to learn more about the art. Those are the kind of students you want on a more long term basis.
Although I realise that you must be present at the sports fair, perhaps the best thing to do would be to let the demonstration wait until your sensei can join you. You don't neccesarily have to let your dojo's terms follow the university term. At the fair you could make some kind of display - perhaps a video of your sensei doing some techniques and a few plates with pictures, text and an announcement of the comming demonstration. Then you can concentrate on handing out pamphlets that people can read at home and you can dedicate your time to talking to those who looks seriously interested thereby getting your key points acrosse much better.
Just a few thoughts - hope they do some good