Well, rankourabu gave you directions to the red light district, which is indeed filled with colorful things and people, but may not be what you were hoping for.

In earlier years, I've spent my share of time there and in the coffeeshops, these days I do actually find myself passing through town on business sometimes and look for other things to do.

I'll second the vote for the Leidseplein area if the weather is good, I definitely like just sitting in a cafe but I would actually walk up there. On the way you will probably pass an Albert Heijn supermarket and I always stop and buy a few packs of stroopwaffels.

There are, of course, the major attractions and museums, Rijks, Van Gogh, Rembrandt, Anne Frank House, etc. There are also some modern museums I've been meaning to check out that a google search will find you.

I hope that helps, but to be honest, I feel like my knowledge of the city is pretty limited too. I'm doing a quick overnight there next week and would love to find some new things to see.