Let me clarify.
If you check out their website there are 3 relevant types of karts:
Little Thunda;
Big Thunda; and
Twin Engine Pro Karts.
Not even going to bother with the Little Thunda, for obvious reasons.
The big thunda is the "slow" kart I mentioned - IMO.
The twin engine karts are the fun ones - fact
For our purpose, the Big Thunda on the Jack's Hill Climb is probably the best 'bang for buck', they are slower than the twin engine karts, but still fast enough for everyone to have a fun enjoyable time.
It's a bit less serious (no race suit etc) for people who just want to put put around, and a more technical go kart track than the larger raceway so still some bragging rights for the winner.
I think the best option is Midi/Maxi Grand Prix, decent amount of track time without being too much.
Maybe schedule it for 10am so we finish up around lunch time and then cruise to food?
There are other places Matt, but Warren's is probably the easiest to organise.