At Safeway they sell packages of food-grade flowers in the produce section, sometimes I get them for the fids, but mostly they just throw them around. I have also heard of people growing there own potted rose bush and keeping it on the side of the cage so that some of the branches poke into the cage. I know of one cockatoo that LOVES to shred the branches and eat the rose hips.
Flowers (for food) are a good addition to a bird's diet (not necessary, but still fun!). Just remember that unless you grow it yourself with no fertilizers and pesticides/herbicides, you have to buy food grade flowers.

ETA: I forgot! Phalenopsis orchids (aka moth orchids) are safe for birds to play in, and this time of year you can buy them in any super market (you might want to repot though, they normally put orchid food in the soil mix. This kind of fertilizer is not only bad for birds, but will burn the plant's roots.) Also Aloe is a good plant to have too. It can be used for a number of things!
Many have forgotten this truth, but you must not forget it.
You remain responsible, forever, for what you have tamed.
-Antoine de Saint Exupery