For save the dates, what you send out is going to depend a lot on how much information you want to include. You can go for a card, a magnet, or both. Often people will have a longer note written up with more information (hotel information, usually), and then add in a smaller printed refrigerator magnet with a fun design, so that they can include both a good amount of information along with a cute/fun magnet.
There are some design ideas that people love regardless of whether it’s a magnet, a card, or something else:
1) The classic designs that couples love include a calendar of the month they are getting married, a photograph, or both. 2) Sports ticket look-a-like! You’d probably be surprised by the number of couples united in their sports enthusiasm. 3) Boarding pass look-a-like. Some couples go so far as to commission cards with a small tear-away piece, just like some boarding passes. This idea is perfect for a destination wedding. 4) Movie ticket stub look-a-like. Sometimes couples feel like they are putting on a large production with their wedding; This, or another related idea (movie poster advertisement look-a-like) are a way to make some light-hearted fun. 5) Photo strips. A throwback to those times when couples would get into a photobooth and make funny faces at the camera, these are usually just what you would think of- a long strip with a number of photos. If you get a large enough photostrip, a great idea is to hold a different sign in each photo to write out “save”, “the”, and “date”. 6) Cute cartoons and/or caricatures. Couples are often too cute; Some aren’t afraid to admit it, including cartoons or personalized caricatures on their save the date design. 7) Stickers. Many people maintain a large calendar to keep track of their upcoming events. Consider including a small 1 or 2 inch sticker with your save the date mailing, for your guests to stick on their calendars. 8) Do-It-Yourself intensive projects. For example, getting a number of small door keys, and writing on each one. Or, a small number of sea shells, and writing on each. (Each item could only accommodate a bit of wording- you might see "Save the date", and then "the date", "bride's name", "groom's name", "invitation to follow"). 9) Something sweet. Tins, small bags, or something else containing a sweet, along with a note or sticker that declares your event. 10) Scrolls mailed out in round tubes. Sometimes this might be a simple piece of parchment type paper printed- but you might see things go a bit more upscale with embossing, or even added handles to the ends of the scrolls. 11) I've actually seen this more than once- couples writing their save the date information on their hands, feet, (or foreheads!) and taking a photograph to use as their save the date.
When thinking about your save the date design, just remember that it does not have to match the invitations you probably have not picked out yet. If you and your fiancé are both musicians, get a design with musical notes or instruments incorporated; Have some fun, and make it personal.