Obviously the RLD is a must-see and at this time of the year it will probably be best to walk round there around 8/9pm, in the summer the atmosphere is good and it's better around 11/12 o'clock.
The Heineken brewery can be a bit of fun and might be worth a visit if you like the beer.
As LIL says, Leidseplein is good for street acts and there's plenty of restaurants and bars down there. Beware the casino just around the corner - don't go spending all you LOS holiday money in there
Rembrantplein was my favourite for the night time, again lots of choices for bars, restaurants and nightclubs.
The museums are worth a visit and are pretty much located all together.
Finally, there's the canal cruises which go around the inner areas. Not great but you get to hear about a lot of Amsterdam that you probably won't get to see and the boats do go down the canals in the RLD
