Me, when I started back in 2007, I started with making clothes, accessories and avatars which did not exist back then,
like traditional chinese clothing (literally hundreds of those, all styles and dynasties, also some minority outfits), dancing outfits, skating dresses, ballet shoes, etcetc.

I made a lot of fantasy avatars, buildings, loads of stuff.
Mermaid things were on the side, I was into the serpent/dragon/fairy market.

That took me several years, but at one point, I had simply made everything I was interested in making, so I'm only seldom logging into SL these days.
Usually I check what's new (and some of the things I made are STILL not available on the market, or in a version I don't like), maybe chat with some friends if they're on.

If they are on and have nothing else planned, we might meet somewhere, go RP, dance, or just talk.
I'm not a big partygoer, but there are clubs with dance and live music all over SL.

There is also the possibility of investing a bit of money in SL, then you can rent a place, furnish it, and hang out there, which is probably neat, but I lack the time and interest for that right now with my busy RL.