It seems that bananas are either green or browning fast. If you want to have a little more control over the ripening process, separate your bananas, rather than keeping them together in a bunch. This is because of the way ethylene gas (the catalyst that causes bananas to ripen) works: the connected stem makes it easier for bananas to receive and transmit this gas, causing them all to ripen faster. Separated, bananas will ripen much slower (and can be more easily employed to ripen other fruits as well).
As mentioned earlier, ethylene gas can signal a fruit or vegetable to start ripening. Some kinds of produce are more sensitive than others. Cucumbers, for example, are fairly sensitive. As such, cucumbers will quickly spoil if they are kept too close to other fruits and vegetables releasing ethylene gas. This means you’ll want to keep them away from your apples and bananas. While you can stick them in the crisper, they don’t need refrigeration, so you could leave them on a counter; just keep them away from the fruit bowl.