You can get great results from combining two or three different types of plant (e.g. Lobelia, fuschia, busy lizzies, pansies, pelagoniums, ivy geraniums,etc) or different colours of the same plant.
If you have the space, its also fun to try colourful fruit (e.g., strawberries, peppers
or tumbling tomatoes) - you get nice displays and something to eat too!
And dont forget - you will need to be careful about making sure they are properly fed and watered (you can get special water-retaining compost for plants grown in containers).