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.
Depending on the type of wedding you are going to have, you will be able to design or to find the perfect wedding invitations. The trick is to take your time and not to fall in love with the first wedding invitations you see that has a low price. There are hundreds of samples online and it only takes a few hours to look through all of them. Therefore, you need to make sure that you go through everything and only then decide upon one particular type.