Cool is subjective - what sort of cars (suitable in this case or not) would be on your cool list?
I think you'd be looking at Golf/Focus/Astra-sized cars to get the required space in the back, and cars are losing some of the quirkiness by that stage in the market. You could try something like a Citroen C3, C4 or Berlingo (the latter is quirky in an ugly sort of way, and six seats).
In terms of cheapest fuel costs, obviously more recent cars and engines publish better figures, however you're losing more on the car's value - if you're doing small mileage, fuel isn't as significant a cost as a lot of people think.
Best mpg figure I've seen - 94mpg; the car it's attached to? Porsche 918. Cost? £750,000. MPG isn't the be-all and end-all.
My DH has a Skoda Roomster. He thinks it is very cool! It is very good on petrol, and has a glass roof, which is lovely for the kids in the back. The best thing about it though is the fact the heated seats (front only) get really hot!
Prices start at about £12000 I think, have a look here.