My favorite wedding that I have ever been to was a fair wedding. Complete with bales of hay in the shape of their initials, mason jars for drinking, a wedding pie instead of cake and kettle corn between the ceremony and reception and a fireworks finale. Amazing wedding!!!
Many people use themes that correspond with the time of the year -- fall, Valentine's, America (4th of July), etc. You could also use a theme of something that is important to you -- seashells if you are from the beach or something from your college or even something from your career.
Most people go with the season as far as picking colors and such. If you have it in the spring then you would have more pastels and you could have it outdoors with flowers. If you had it in the fall leaf changes and natural colors like red, orange that mimic the leaf changes.