Cake decorating can be a tricky skill to master -- too many piping bags required, and fondant scraps all over your kitchen, and food coloring stuck to your hands for days. It's easier than you might think to make a very impressive cake, without needing to have lots of decorating practice, and here are my favorite ways:
Fresh flowers: Find some sort of edible flower (impatiens, snapdragons, nasturtiums, and lots more are edible) and carefully arrange them over the surface of your frosted cake. Pick one variety, like I did in the photo above, or handfuls of a few different colored ones. This takes no time at all, and is a helpful trick for covering up not-so-beautiful frosting.
Swirls: If you're not confident making gorgeously swirled buttercream frosting designs, use seven-minute frosting instead. Made from just egg whites and sugar, this frosting whips up into an airy cloud (like a meringue batter). Swirl it onto your cake in dollops with a spoon or offset spatula -- it'll look pretty regardless of how messily you apply it.
Naked cake: This is a thing now, even though it makes me laugh every time I see it written. Keep things easy and forgo the frosting exterior entirely. Just fill your layers with jam, or lemon curd, or whipped cream, or frosting. Then leave the outside of the cake as is -- a topping of fruit will make it look even fancier.
Powdered sugar: Most cakes will benefit from a dusting of powdered sugar. Especially for plain, unfrosted cakes (like yogurt cakes or almond flour cakes), this transforms an ordinary cake into something planned and pretty.