I suggest not sticking together as one large group the whole time as it'll be a logistical nightmare - last time I was there was in a group of 20-odd and it did my fucking head in. As did the Ecuadorian shrooms, but that's another story...
The Van Gogh museum is a must if the idea of culture isn't anathema to the trip. Don't waste money on the sex museum or cannabis museum, they're both shit.
The Winston, just past the north end of the red light district is where hip Dutch kids hang out (or at least it was 5/6 years ago when I was last there), IIRC, Sunday nights are the best time to go. It'll be worth having a look what's on at the Paradiso too - if nothing else, it's a fantastic building. There's a sweet record shop just across the road from it too.
Coffeeshop-wise, most of the ones in the red light district are horrendous tourist traps that all sell the same overpriced commercial weed. Stock up at the places that grow their own instead, like the various Greenhouse shops, or my personal favourite, Amnesia, which is out towards Jordaan and tends to be frequented by locals rather than tourists. Oh, and it's a bit of a cheesy Brits-in-Amsterdam thing to do, but getting breakfast at Barney's up near Centraal Station is totally worth it.
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