Proper etiquette for an open house starts before the party, with the sending of an RSVP. On the date of the event, be sure to bring a small gift for the host or hostess and stick to the areas designated for party-goers. Depending on the location of the event, dress in comfortable layers in preparation for indoor and outdoor temperatures, if necessary.
At the party, always be polite. Be sure to ask beforehand if kids are allowed, rather than just showing up and expecting them to be accepted. If there are any other questions, they should be asked prior to the date of the party, if possible, so that the host is not placed on the spot.
For host or hostess gifts, a small fruit basket or other consumable items are normally a more thoughtful gift for this occasion. Also, while cut flower bouquets should be avoided, as it might not match the party decor, a small potted plant in a neutral color may also be appropriate.