I am in Humanities but I also missed the opening weekend at Stanford. Don't worry (or feel bad) for not going - it is common for international students to miss open house.
I have been in Top Three for four consecutive months. Very impressed with myself. I cannot get higher than the third spot though - bhikhaari and newms just wouldn't share.
Yes, I also need to get that thing.... what's it called? ... mmm... oh yeah, A LIFE!
I would send a very short e-mail, thanking for the decision, mentioning the phrase 'It is a great privilege...' blah blah.
You don't need to commit now and no one expects you to. I don't think it is necessary to say that you are considering that offer, implying that you have/will have others. Just 'thank you'.
Yesterday, two rejections were reported for History & Middle Eastern Studies. Both by post. It seems that Harvard reached decisions on that programme, and it is just a matter of time when snail mail reaches applicants. Sigh.