There are a ton of great things to do in second life, but you have to really understand the various reasons why you come in world too.
I personally LOVE the Builders Brewery and when I can I find the time to take a class on building. It really gices you an appreciation for what goes into creating such an amazing place as well as provide you with every level of skills to create your own things.
Also, roleplay should not be overlooked. Just take some time to read a communities set up and rules before engaging. you will find many people willing to mentor and help.
Things like sailing, flying or riding motorcycles can be fun too, there are amazing communities that revolve around that.
Finally, talk to people. Be polite and find out what they like as well. Find something you may not get to do in your first life, but have always wanted to and do a search, you will find a communtiy for it!