Mbudia island is a must. Take the ferry, is a 25 minutes ride, and you will be in paradise!
And once you are back, next to the ferry gate there is a hotel called Azure Butique Hotel, go for the best sea food in town.

 

Also there is beautiful place at Hyatt regency hotel at the centre of Dar es salaam city

 

Before a period of seven years, I visited there. On Yacht Club Road, I particularly enjoyed myself. There's a wonderful tavern called Cape Town Fish(I guess that's the name) that takes credit cards and is generally pleasant.
I'm not sure where you'll be staying, but I'd suggest walking from Barack Obama Drive (coast) back to the city centre, passing the embassies. In the wealthier part of downtown, there are a few fine restaurants and bars.