Help clear the table. If your host/hostess doesn’t want you to help, they will see you made the effort.
Don’t hide in your room except for the meals. Bochrim are expected to attend a minyan. Girls, if not going to shule, should be out of their room(s) and spending some time with the host family.
(For visitors in Israel – if you are going for the entire Shabbat, don’t expect to shower when you get there. If you do shower, make it quick! Remember that water in Israel is dear, precious, scarce, and expensive!)