Hi there,
To start off your Seattle day trip of the
downtown area you could begin at
Pike Place Market, which is one of the most famous landmark in Seattle.
At the Pike Place Market, you can find carefully prepared foods from all around the world, handcrafted art and jewellery, etc.
From the market, you can head to the nearby waterfront via one of the free buses and visit an IMAX theatre. From the waterfront, you can catch a bus north to see the
Space Needle and the Experience Music Project Museum.
However, if you want to take a Seattle tour then you would want to start at Westlake Center at 5th and Pine where one can buy tickets for the Ducks Tour and take a land and water tour of Seattle.
Hope this helps
VacationBuddy