Your save-the-date cards should be as functional as they are fun. Of course, clearly include your names, wedding date and location. If you haven't picked out a wedding venue yet, that's OK, but definitely add the city and state where your affair will be held.
Also, be sure to write guests' names clearly on the address label, similar to the way you'll address your wedding invitations later. It's fine to be less formal with save-the-dates, but including each name will make it clear whether or not a date will be invited. If children are omitted from the address label, this will give your guests plenty of time to arrange for childcare. Guests may RSVP to you early, letting you know that they won't be able to attend. If this happens, make a note, but you still must send these people wedding invitations. Guests' plans can change, and you still want them to feel welcome to share in your special day!
If you plan to make a wedding Web site, supply guests with this address on your save-the-date card as well. Creating a Web site is a great way to give guests tons of information about your wedding ceremony and reception locations, the weekend's schedule of events, local hotels with rooms blocked off for guests, and fun facts about the bride, groom and wedding party. While you shouldn't include registry information on your save-the-date or wedding invitation, it's OK to reserve a space for this on your wedding site.