the best place for grass, although not that special inside is Central Coffeeshop opposite the railway station, if dthey have it try their Peacemaker - comes with a warning on the packet, v strong.
For hash go to the res light district, aa few coffeeshops there that do 50 and 100 euro deals, even though they should only sell you 5 grams - specials on afgani and indian hash.
then for the surroundings you are spoilt for choice, check out the Grasshopper, opposite station lit up with green lights, can't miss it; the Bulldog Cafe, lots of Brits (hence Bulldog), sensi seeds coffeeshop near the hash museum, does not sell cannabis but you can take in your own and chill to break up the walk round the red light spots and museum.
there are coffeeshops on boats, trippy bars, rasta coffeeshops, cheech and chongs, fucking hundreds of them - have something for everyone.
you will love it my friend - went 3 times last year! what hotel are you using?
the people are polite, friendly, and most speak engilsh, and have never seen any hassle over there. The law says you can only carry 5 grams, but if you even have an ounce they will not bother you -unless you have pissed them off.
have a good time