places to see from a culture point of view are charles bridge (you'll see it loads), prague castle and one less well known in my experience is 'the loretta' (think thats right) - a beautiful small church/chapel with an amazing collection of bejewelled items.
If you're on a stag night, and you want to squeeze in a bit of culture, try the very interesting and quite shocking medevial torture museum - it's somewhere in the city centre.
As you'll spend most of the time drunk I guess, tell all in the group to carry wallets in front pockets or better still money wallets, pick pockets are rife.