Plants are an excellent and inexpensive addition to any patio. The hardest part is figuring out what you want to add. You could go with a flower selection, vegetables, plants that will attract butterflies or birds, exotic additions, or even just a section of grass amidst the pavers for the dog to roll in.
Again, before making any additions to the patio, plan what type of plants you want and where you want them to go. Also, spend time watching the sun track across the patio, and consider the amount of light and shade it gets. Why would you want to see sunshine plants in the shade? Also, take into consideration how you'll maintain and water the plants. Is there a spigot nearby, or will you have to walk the water to the plants? Can you plant directly in the soil, or do you need containers? If so, how much soil will you need? How will you get this? Where will you store the fertilizer, watering cans and small tools?
Once these questions are answered, you can start looking for containers. Thrift shops are good locations, as well as Internet want ads and yard sales. Don't pass up the odd, either. Maybe that old baby buggy would look good with a few potted plants nestled inside. An old, metal baker's rack can be set in a corner, and you might fill the shelves with plants and maybe some tacky knick-knacks.