Safari from Nairobi to dar es salam

Hi Julius,
I wonder whether you will get a tour operator that does this kind of routing on their safaris. Personally I cannot suggest any. There are no parks close to Dar es salaam and as such itineraries similar to what you are looking for are hard to come by. However, there are several options that you may consider that will see you get to Dar es Salaam and have your onward boat trip to Zanzibar or even Zanzibar first then Dar by boat.
Dar es Salaam is closest to Mombasa from Kenya but at the same time a great deal distance-wise if you opt to travel by road- in excess of 340 miles or 10-plus hours drive.
My suggestions..
Nairobi-Amboseli-Tsavo-Mombasa-bus to Dar es Salaam
Nairobi-Amboseli-Tsavo East/West-Mombasa-Fly dar es salaam-most flights via Zanzibar or Nairobi!
Nairobi-Amboseli-Tsavo-Tarangire-Manyara-Ngorongoro-Serengeti-Arusha-fly to Dar es salaam, you will require quite a number of days for this itinerary.
Nairobi-Amboseli-Arusha-fly Dar es salaam

Options are quite varied all depending on your budget, # of days you have and also your particular interests.

You might consider reversing the boat trip to start from Zanzibar and on to Dar es salaam as almost all the flights will land you in Zanzibar first.

Would suggest you contact companies offering Kenya and Tanzania safaris and give them your skeleton itinerary, they will be in better position to give you the most suitable itinerary for the same.
for a start, you may consider e safaris http://www.e-safaris.com or Hm Safaris http://www.hmsafaris.com
I have had good experiences with both. Other LP experts will give you alternatives and suggestions.
Good luck!

First though, take a look at the map and the journeys and distances involved. You might be better off finding a tour company who will take you on safari more locally to Nairobi and return to the city, before catching a bus, or train to Dar - it would definitely be cheaper!