First, for the most part, play it simple when it comes to the suit. Stick to dark or neutral fabrics to keep it as basic as possible before adding the following flair.
Many different florists and Etsy vendors make fun and unique boutonnieres with different elements such as small toys, candy, fabric, and miscellaneous baubles. They have a big impact, so if you go this route, keep everything else clean and simple in order to avoid looking like an escapee from Barnum and Bailey's.
Keep the suits the same, but differ the neck wear. Use bowties in different fabrics or use the same fabric for different style ties (bonus points if you can get someone to wear an ascot). You can also have your ushers join in the fun with their similarily-colored suits.
Keep the colors muted, but wear a textured suit fabric such as linen, seersucker, corduroy, or velvet. Fortune favors the bold, right?
Match your groomsmen attire to your wedding's theme. Texas ranch wedding? Try Wranglers, boots, and sports coats. Beach wedding? Forget the suits and think linen pants and pressed white shirts. Going preppy vintage? Layer sweater vests under corduroy jackets (dont forget the elbow patches).
Add accessories to your attire that fit your wedding's location. If it is more formal, add panache to your suits with pocket squares. For a winter wedding, have the groomsmen wear matching scarves or use the same umbrellas if the weather decides to be difficult. And remember, the accessories can always be fun items that the groomsmen only use or wear at the reception! No need to worry that fedoras or sunglasses are going to ruin any ceremony pictures.
Looking for more groom and groomsmen ideas? Start here.