Old, Cheap
Fun is knowing that if you wreck the car, you can afford to just walk away. My $200 Dodge Omni was fun, off road, jumps, it could do everything, as I was ready to just walk away if it was damaged. It never gave up, though.
Fun is also, for me, maneuverable. I like how my bike can explore and go places in the city my car cannot.
I also find open air fun - like a motorcycle. Maybe a convertible (I've never had one).
Maneuverability is nice, though not too exciting.
Outrageous speed can be fun, though of course very dangerous. What speed is too fast for the handling depends on the vehicle, of course. In that sense, go-karts are more fun for less danger than high performance cars.
Comparing the Yaris, Mazda2, Spark, Fit, and Mirage on "fun to drive" seems pointless. None of them are "fun to drive". All of them are useful for getting to fun places more often (as you have to spend less time at work to pay for them and the gas compared to bigger, more sporty rides).
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View my fuel log 2014 Mirage SE 1.2 manual: 45.0 mpg (US) ... 19.1 km/L ... 5.2 L/100 km ... 54.1 mpg (Imp)