Is that the only time you're going offsite? If it is, 2 mini-van taxis (but you'd want to confirm how many they can hold) would make most sense to me. I can't remember the taxi price but, for me, a taxi both ways was cheaper/easier than a single-day car rental plus IOA parking.
However, if it's multi-day, you'd probably find a cpl mini-vans would be so much less hassle.
There's a few car services attached to the board as sponsors so they probably have links. You could try getting a quote from them as well.
Hope that helps.
