Gosh, there are so many things to do in SL... it all depends on your personality. I've listed things that I love to do below. I'm an explorer at heart and love good conversations (ones that involve a variety of topics, whether deep or light).
-Look at the Events going on all over SL. You can find them via the Destination guide, doing a search inworld, various SL blogs etc. There are live events, fundraisers, discussions, story time, poetry and so much more.
-Go to game sims. I love to play table games such as Greedy, Parcheesi, Skippo, UNO, Yahtzee. You can find them all at one of my fave game sims, Haven Games: http://maps.secondlife.com/secondlife/DarkHallow/64/192/2
- Go on SL hunts. If you've never done one, what it involves is looking for hidden things and getting free/low priced stuff. Kind of like a treasure hunt. Learn more about sl hunts here: http://huntsl.com/
- Amusement parks and carnivals in SL. Always fun!
- Go boating/sailing, jet skiiing, surfing
- Visit various SL cities like Paris, London, NY and more
- Learn something new....learn to build, script, make clothes etc. You can find sims for that. To start off, here are a couple:
Builder's Brewery http://maps.secondlife.com/secondlife/Builders%20Brewery/128/193/23
Caldeon Oxbridge University http://maps.secondlife.com/secondlife/Caledon%20Oxbridge/72/140/31