is there any problem with going outside before you have lunch, taking off your shirt and running a mile? this should allow for at minimum 5 minutes, at maximum 15 minutes of sun exposure.
In the US, there are laws against this kind of thing, although they typically exempt nearly half of the population. Worse than the laws, though, are the social sanctions.
i assumed the half of the population who weren't allowed to take off their shirt would wear some kind of minimal sports covering. even an a-shirt or something similar would still expose one to a sufficient amount of vitamin D, perhaps with a few more minutes exposure.