I leave the leaves in the tank until they have reduced to skelatons, this gives the leaves plenty of time to develop a biofilm over them that the shrimp will feed off as well as the actual leaf.
With the Banana just cut a bit off the fruit and weigh it down with something like a fork like you would for weighing down zuchini. I wouldnt leave the banana in the tank too long maybe only between 1/2 and a whole day. If you have lots of shrimp in the tank it wont take them long to discover the new food and from their reaction oyu should be able to work out what time frame will suit them.
Something else many of my fish, shrimp and snails all like are nori sheets, the seaweed wraps that go aorund sushi.