Could you be
FLOUR and FLOWER?
You can see me at a party or on the ground
At a birthday party, there is always cake, which has flour.
On the ground, flowers can grow.
Whichever one, it makes the same sound
That defines what a homophone is.
If you find me on the ground you can pick me up
If you find me at a party, you probably can't
True for the first part.
True for the second part, but not technically. Slices of cake containing flour can be picked up with a utensil (you yourself can't probably pick up the flour!), but obviously, the purpose of the action is to not actually pick it up; it is to eat the cake!
Flour and flowers are not balloons, rocks or bases... but they are not really litter, either. I guess a flower could be litter, as the archaic definition of litter is the choice of bedding for a horse (that of which can include plants).