well, there are always colorful plants like geraniums, and other blooming plants. Build some planter boxes with left over material and set off the deck on the ground and this way will save space and the deck will look bigger.
Other ideas are railings with neat spindels, then there are benches built of the same material, then even an arbor over part of the deck.
What you want to watch out for are making too many things that go on the deck therefore making the deck smaller and less space to get around. A nice table with umbrella attached is nice also.
I would go with landscaping around the deck with plants that are colorful.