xdh

I've walked Howard Street often during the day and always felt happy although it can be quiet with few people about until you start to get towards the Moscone Center I'm not sure that I would walk it in the evening or at night though.
I would stay away from the cross streets 7,6 and +/-5 even during the day and the streets running parallel between Market and Howard unless you had a reason to go that way.
If you are in The City for some days a multiday Muni pass is very good value and as erdehoff said means you could take the F line which is a fun experience.

Fleet week is also on that weekend with practice on the Thursday and Friday and the full display over the weekend, lot's of information on the web and you can visit the ships if you have the time. I've always enjoyed it on the 3 times I've been.

DMF