Visiting Barcelona in early March

insights about public transportation; under the radar fun things; etc.

It is extremely popular there; under radar, fun things depends on your interests age range etc.
https://www.barcelonaturisme.com/wv3/en/

There are many fun "under the radar" stuff to do and see in Barcelona, may I ask what things specifically interest you? How much time will you have?

I recommend stopping at the Blacklab Brewhouse & Kitchen.

The list of top sights at the web link in the post above mine is somewhat limited. I would urge you not to depend totally on it.

Well, as far as I'm concerned any list that omits Casa Mila/La Pedrera and the Sant Pau modernista site [oops--forgot MNAC!] while including the Colon Monument is a bit suspect. Any guide book would be a better source for a list of sights. The event information is probably of more use.

If you like wine, go to a wine shop. We found one near our hotel (the Hotel Colon) called Vila Viniteca that was having a tasting on Saturday afternoon and had an employee who was fluent in English.

If you like boats, go to the Maritime Museum.