I agree with ze'ev and the others that your demonstration should reflect as closely as possible your normal training and your authentic skill level. That's the rule for our organization, so that there should be no 'special' preparation for a demo; you should be able to perform your aikido 'as is' even against an unexpected attack, much less an anticipated demonstration. If you are still wearing your white belts or at least non-black belts then the general public will probably understand and appreciate your honesty.
However, I also think back to what first attracted me to aikido, and I think it was mainly due to its unique philosophical attitude and its clear articulation of martial principles that were absolutely in agreement with its philosophy. I'd trained in taekwondo before and aikido cleared up a lot of question marks I'd had. I think it'd be great if you could coordinate your explanation of the philosophical aspects of aikido along with the physical aspects. What would be good about this is that you wouldn't necessarily need to even demonstrate techniques like shihonage, kotegaeshi, etc. An unexperienced onlooker seeing aikido technique will just see people grabbing wrists and arms and people flying and falling and won't have much idea of what is happening beyond the 'wow, how do they do that' response, which isn't very educational.
For example, you could talk about aikido's emphasis on natural movement and posture and the power of that movement and then demonstrate this by calling on members of the audience to try out 'unbendable arm'. One neat trick I also saw was having one person lying belly down on the mat with their arms wrapped around a standing person's foot to try to keep them from walking away. As long as the standing person is concerned with the foot being held he'll have trouble moving, but instructed to completely ignore the hold, the person on the mat will be dragged along with no problem at all.
Then you could talk about how aikido does not clash with the force of an attack but how it meets that attack at the point of least resistance by either irimi or tenkan and unbalances the other person at the very moment of contact. Again, you wouldn't want to show technique, just show how the power of the attack is dissipated and the center of the attacker displaced.
After this is made clear, then you could finally demonstrate that technique is executed against an opponent who is already unbalanced and compromised and in itself is almost superficial to what comes before. I think this would do a much better job of communicating to a spectator what aikido is really about even more than black belts in hakama doing spectacular breakfalls.
Anyway, this is all just a suggestion and I hope you have a really successful demonstration whatever you eventually decide on doing. Good Luck!