I am Israeli and will be in the US for Simchat Torah. I know in Chu"l they do 2 day chagim. Since I only do 1 day, I am not sure what to do because we will all be at a hotel, so there will be marit ayin as I leave the hotel. What can I do in this case?

Shalom, I hope your trip is a beneficial one, and that you return speedily and safely to the Holy Land. Israelis outside Israel on the second day of Yom Tov must pray and put on tefillin etc as they would in Israel (in private). Havdallah must be said at the end of the first day of Yom Tov. However, if you are in a city where there are other Jewish people (as opposed to camping in the middle of the desert, for example), then you may not perform any forbidden labor (melachot) even in private. This is the opinion of the major poskim including the Mishna Brurah. So my advice is to spend a nice slow day in the hotel, learning and relaxing. If there is a synagogue nearby, you might want to put on tefillin in your hotel room, then walk to shule and join them in their Simchat Torah - it's always a mitzvah to join Jews in their celebration of the Torah! Chag Same'ach