Dar es Salaam is Tanzania's largest city and the country's commercial capital, best known for Kariakoo Market (East Africa's largest open-air market), the National Museum of Tanzania, colonial-era architecture along Sokoine Drive, and its role as the mainland ferry gateway to Zanzibar.
Don't let the office towers fool you — Dar is really a coastal Swahili city that grew up fast. Come for the markets, the seafood at Coco Beach, and the dhow-lined harbour; leave for Zanzibar on the morning fast ferry two hours later.
Most travellers give Dar 2–3 days at the start or end of a Tanzania trip. That's enough to walk the CBD, get lunch at Slipway, and set up your Zanzibar or Selous/Nyerere onward journey.