Well this is a funny experience and felt like I must share bearing in mind its a tourist office managed by the government...We came across this when visiting the sight and decided to take their advice and book us a tour guide from them to take around the city (he was superb and did a great job with the tour). We have wanted to pay with $100 note for the tour and asked the guy behind the desk if he can give back change, he said yes, taken it and then suddenly said he cant. Fiddled around under the table for quite a while which was pretty suspicious give back the money and has taken smaller change. When we were about to question the tour guide has arrived and we just had enough time to look at the money to see if its legit which looked like it was.When finally could discuss about the whole thing and how odd it was with my partner we looked at the money took it to our hotel and realised he has given us a note from 2001 - something that they dont take anywhere in Tanzania!!! In short he has pretty much stolen from us as unless we are not prepared with enough cash or credit/debit card you cant pay with such note and your only chance is to exchange it there for less shillings....
There is the oficial tourist center suppoerted by goverment. They can also provide tours (Spice Tour for USD 15), car rentals and many other!
Fair, to the point, clear information, can even pay partial and organize the things around - if you are not clear, dont want waste time go there