When I first got into a kitchen, I was putting fruit carvings with everything lol. Then I started using real flowers, leafs, etc. It was because I thought it looked so cool, and because I could do it and others I worked with couldn't I simply did it as a way to show off. Now I won't do either. It looks super tacky to me. If you're going to do a fruit tray, then of course those things will compliment the tray itself and go good with it.
It all comes down to taste. Don't put something on a dish that wouldn't go with what you're serving. Strawberry roses with steak. Edible orchid flowers with an Indian cuisine inspired dish.
As another chef has said in this thread, you could use something such as a mushroom for garnish.
It's something simple, that may go with your dish, compliment different ingredients, and is art on its own. Think of it like this. Take all of your ingredients in your dish and ask yourself, how can I change the shape of that and compliment the dish? Maybe julienne some different colored peppers and let them soak in ice water so they curl up. Or maybe change the shape of a potato such as making it into a cylinder, or cubes with a ball inside (google it).
Every time you change the shape and texture of food, it is no longer the same. It will cook differently, and maybe even taste different depending on the way you incorporate it. So always take that into consideration. You may want say a fish and chips dish to look more unique. You may want potato seaweed. So you fry off some Latkes. It now looks cool, but you've changed the taste too.
Sauces. I can think of so many different variations to apply sauces so it looks unique.
I've moved on from garnishes for the most part. I let the food speak for itself by the way it is presented on a plate and what else is on the dish complimenting it.