I second the Downtown Marriott. It is very large, a convention hotel, but for all-around location It's the one I'd choose for a first-timer in SF. Also, my experience is that this hotel is very accomodating w/regards to upgrades for Plats. If there's any downside to it, it is that it's a huge convention-oriented hotel so you can be in with scads of people all competing for use of its facilities.

Relatively easy walks to the Cable Car turntable at Market & Powell and to the Ferry Building (where you could utilize light rail to get to Pier 39 the Wharf & Ghiardelli Square). And, there are great views from the cocktial lounge at the top.

When the weather's nice the ferry to Sausalito is a must-do.

I don't know if they're still doing it, but at one time, July 4 was when they hauled out the vintage streetcars and ran them above ground on Market.

You also might want to consider renting a car for one day - Muir Woods is awesome (and it's another option in terms of getting to Sausalito via the Golden Gate Bridge.

BTW, you might find this site (for SF visitors) helpful.