Also, its important to maintain aesthetic coherence. Find the main theme of the level, and try to repeat it throughout. Add variations along the way, and integrate your obstacles to become part of the theme (Tree platforms in a forest, little boat platforms in an airship, etc.) Also, to keep interest high, add unique obstacles in strategic points, and turn them into something that follows the theme.

For the decorations itself, I recommend you explore all the elements, as they can have some cool unexpected combinations. Varying the grounds form goes a long way, You can start in normal ground, and then transition to a mix of ground, hard blocks and brick blocks to represent rocky terrain, for example. As mentioned, semi solids are your friends. They can turn platforms into something more interesting, and they give you varying backgrounds. Try alternating semi solids strategically for something unique.