3-4hours, it’s very big, many things to do to see, no rush, relaxed, take your time, enjoy!!!

i will go for Ueno!

It definitely Free.

It takes 1 hour each way from narita airport to the city. That's 2 hours gone already and it cost $25 each way on the airport express train. At the train terminal, you would have to take a taxi or subway to your site. You would have a two hours only to see one site. Not worth it in my opinion. Best to see something at or near airport.

I can't compare because i didn't visit San Francisco yet. Ueno Park was very nice especially during sakura (cherry blossom) end of March April. There's a lot to see in Ueno Park like zoo, museum etc. Don't missed to visit Ueno if you are in Tokyo.

the season for cherry blossom is over for now, if you are keen for sakura season then visit Tokyo at the end of March

8th of April.. yessss

Ueno Park is the most 'user friendly' place so to speak. It is astonishly beautiful no matter what time of the year you visit. PLUS! Galleries,Museums, Temples and Shrines. There is also a Zoo ... I am not intersted as caged animals are against my believes, but some people enjoy it. So, I am not in the positin to make comments about the Zoo as never entered. The park is spectatular any time of the year you visit. You can easily spend a whole day there if so inclined. Lotus lake is to die for. Don't get me stared as I could go for ever.;-) Absolutely magcal place no matter what time of the year you visit. Enjoy!

I think it is too early for cherry blossoms in February. Maybe April, but check with youir travel adviser.