I have no idea what the first things were, and there have been so many great things I can't think of a single favorite, but I can mention how I arrived here.
I used to read slashdot religiously (from ~2003-2008) and by default there was a box along the side of slashdot that was a feed of boingboing's posts. I was clicking on those fairly regularly since 2005 or so, and eventually I realized that I was basically just using slashdot as an RSS reader for boingboing.
My participation ebbs and flows, but boingboing constantly provides so much stuff that I think is really great that I'm going to go with "everything" to answer the question, even the posts featuring stuff I actively dislike or are indifferent to and/or don't understand the appeal of. I'm glad those posts happen, because often eventually I come to understand - boingboing manages to introduce me to more new, sometimes challenging, things than any other source by not being an echo chamber like most similar sites.
Actually, the thing that spurned me towards participating initially was something Maggie posted about some new research finding regarding mountains or something, which is my research specialty as an academic geologist. IIRC, my comment was factually incorrect and poorly stated
re: podcasts, the early Gweek with John Hodgman was really great. I've only flown Virgin once, shortly after BB TV came out, and everything on it was great (though I wish there had been a new selection of stuff for the return flight).