A fish pond poses its own set of considerations which are vastly different from an aquarium. Therefore, decorating it must be planned and handled differently as well.
The first issue of a fish pond is the visual perspective. An aquarium will be viewed from the front and sides. Therefore, when designing an aquarium theme, you would place ornaments and arrange them for this perspective. However, fish ponds are viewed from the top. The ornaments that you place inside the pond needs to be those that are visually pleasing when viewed from above. Placing an ornament that has intricate details on its side defeats any decorative purpose.
The second factor also has to do with the visual aspect. Depending on how deep your pond is, placing low lying decoration might not be a good idea. This is because the deeper the pond is, the less light the bottom gets and having decor lying low at the bottom would make it almost invisible when viewed from outside the water. This is one of the reasons why most fond designers incorporate decorative items on the outside of the pond, rather than submerged inside.
One last issue would be the weather. Unlike an aquarium which is placed in the comfort of your home, outdoor ponds are exposed to the fury of nature. The items that your choose to decorate your pond with must be created to withstand harsh natural conditions. These include the scorching sun, the pouring rain and in certain regions, freezing cold. Decor that is not designed for this will start to fade and crack when exposed to the elements.
The key here is to first keep in mind the three major considerations above. Make your selection of each individual item based on these factors.
Pick out a theme that is suitable for an outdoor theme. Some really cool theme ideas include Fairies and Mermaids. There are dozen of such ornaments that you could get that have been designed specifically for an outdoor pond. You could also use figurines of animals that can be found by or in the water such Turtles, Herons, Dolphins, Crocodiles and Hippos to name a few.
An awesome idea is to try to get ornaments that actually look like they are interacting with the water. This would include such figurines as fishermen, little kids tinkling into the water or maidens with vases. Many of these have pumps that flow the water through them and pours out from them back into the pond.
Apart from rocks to provide cover for the fish, you really do not need any submerged decor. Having little or no decorative items inside the water will make it a whole lot easier to clean.
Being outdoors creates an ideal condition for plants. The plants should receive sufficient sunlight and typically will thrive in the water. Also, placing plants inside the pond has a lot more benefits other than just being visually pleasing. The floating plants which live on the surface of the water provides shade and cover for the fish. This is extremely important especially when you have fry that have to hide away from the adult fish.
The plants also provides competition against algae growth. By competing for sunlight and nutrients in the pond, these aquatic plants would starve the algae and limit their growth.