A sign-in sheet is the form, on paper or a tablet, that an open house visitor fills in with a name and at least one contact method. It is the single most important tool at the event, because a visitor whose contact you did not capture cannot be followed up with, no matter how interested they were.
Good sign-in sheets ask for few required fields, keep rows big enough to read, and print a short line explaining why the visitor is being asked. Those three choices raise the share of visitors who leave a usable, real contact.