Perhaps there is a hobby that both of you love to do. You can use this 'theme' in your wedding invitations as well. If you both love to fish, you could use fishing line and lures for your invitations, or a hobby of wood_working could provide you with a way to expand your hobby into creating some special invitations. Regardless, if there is a hobby that the two of you share that helps to define you as a couple, don't be afraid to use it to your advantage and choose the best part of your hobby to direct your unique wedding invitation choices.
Is it acceptable to have a unique wedding invitation even if you are planning a traditional wedding? Absolutely. While some people like to coordinate their invitations with the style of their wedding, it is not strictly necessary. If you have a fun or different idea for your wedding invitations, then, by all means, choose the invitation that you want. If you plan on celebrating with a formal ceremony, make sure to mention that on the invitation, such as "black tie requested". On the other hand, if the wedding invitation is off_beat people may assume that your wedding will be a more casual affair unless you specifically state otherwise. Putting the information on the wedding invitation prevents any confusion.