You don't need to own a car to take weekend trips to the country or even a quick trip hauling some gear: just rent one. Here in the US, we have "Zipcar" which lets you rent a car on the spot for a short term; in Europe, I saw "Car2Go" which is probably similar.

>As a visitor, I rarely have a rental car in SF for example. But it means I am going to be pretty much staying in the city rather than taking a hike somewhere for half a day.

I'm in DC, and I've frequently gone on hikes outside the city and seen people drive to the trailhead in a Zipcar.