I've spent lots of time in Amsterdam.
Amsterdam is very safe (if you don't act foolish). Protect yourself by: wallet in front pocket, don't look weirdos in the eye, don't draw attention to yourself, be polite, be confident, etc.
The Dutch are friendly and easy-going and they speak fluent English. I know because I married one.
The best known coffee bar in Amsterdam is the BullDog (
http://www.bulldog.nl/index.html). It attracts tourists and locals. There are several locations throughout the city. The Bulldog is more like an empire than a coffeeshop. They've even got a hotel. My favorite location is in the Red Light District. It's small and cozy. You can definitely sit down and have a smoke and a java. It is located at Oudezijds Voorburgwal 90 (Telephone +31 20 625 98 64) It is the original BullDog, opened 30 years ago.
Onesnowyowl has given good advice. Most every city in The Netherlands has a collection of local hash bars. Still, I think it is not necessary to leave Amsterdam out of fear. Amsterdam is one of the safest large cities in the world.