Only at the most casual of exhibitions should you ever wear jeans. It is always better to look smarter than necessary than it is to look scruffy, so go for something a bit more upmarket. If you don't want to wear a suit, then khakis and a shirt or polo will do.
You should also think about picking out something with plenty of pockets. Your jacket might have a few, but you are going to need all you can get. You will probably be carrying a mini-office around with you in your pockets, so go for practicality over style in this area.
Again, while you want to look your best you also need to be practical. You could be on your feet all day, which could mean you end up hot and bothered. Venues like Cranmore Park have air conditioning, so you should be okay on this front, but it might be a good idea to opt for something lighter over anything too heavy-duty.
Even if your outfit is fully utilitarian, you can still spice it up with some accessories. The key here is to be sparing. You can wear a few pieces of statement jewellery, for example, but you don't want to look like a Christmas tree. Avoid anything too excessive.
Similarly, do not put on too much perfume or cologne. This can put people off, which makes a terrible first impression. A few quick sprays of antiperspirant in the morning will be much better than drowning yourself in expensive scent.
Hopefully, these tips will help you make the most of the next exhibition you have a stall at. At the very least, it should cut down on the time it takes you to choose your outfit in the morning!