I don’t think riding a bike and riding a scooter should be asked as part of the same question. One has very serious tznius issues (the bike, that may or may not easily be dealt with), while the other (Scooter, standing up) is just part of “should a girl be seen outside doing something other than…” kind of thing.
Never mind the driving a car – I get so mad when I hear it lumped into a “tznius” issue… (How much more covered up while outside can one get than being boxed up in a metal transportation mechanism)
The “ripstik”? Now that’s a whole other parsha. THAT is something that I think is the epitome of “untznius” – I hate to see even boys use it, but I absolutely would not allow my daughter to… for those who are not familiar with it, it’s a kind of a skateboard that to move forward you have to move your hips in a gyrating motion (think Michael Jackson!). And yet, I see the kids use it all the time. I think it’s really sad that none of the parents think twice about buying or letting their kids use it – especially older girls (tweens!)