Depends. How am I managing my time?
As I wrote, I did not realize at the time of the first post that this project is completely on the exploratory end of the spectrum. Knowing that now, again, I would never recommend an interview or survey. You need direction for those methodologies (hence the term directional research).
For where the OP is, I would recommend secondary research and observation. But as you put it, I would go with the “casual observational research”. I would not recruit a respondent and follow them for the day. As you and the OP suggested, going to a train or bus or airline terminal where the subjects are unaware of being observed would be the best for time management. You will get the most bang for your buck.
Once the OP gets a direction, e.g. pre-planning activities with children under 5 through a travel site, the interview and survey can come into play. I still think recruiting the “right” person for one-on-one observation is more likely to fail than succeed and would be a huge time sink and doing nothing in that case would be better.
And while the Designing with People site claims you can have an unstructured interview, the Stanford site says,
“While we always must allow room for the spontaneous, blissful serendipity of a user-guided conversation, we should never abdicate our responsibility to prepare for interviews”
I know a handful of people who could possibly pull of an unstructured interview, but they have been doing research for 20+ years and are some the best at it. I have been doing research for 20+ years and I certainly know I could never pull it off well, so I prepare. Having a novice try an unstructured interview would in no doubt result in poor data.