You can put plywood underneath the cushions. There is a support kit you can purchase which is a series of interlocking slats that you custom to the size of the furniture you need to support.
You can get foam cushions cut to fit your existing couch or add fiberfill and plump up the bottom cushions.
I took a fun doodle, cut it to length and stuffed it between the crack between the bottom and top cushions. It seemed to fill the space and the couch was much more comfortable.
What is a fun doodle?
Swim tube available at almost any food market, Walmart, Target even the dollar stores. They come in bright colors, some are thicker than others. Long tubes made of polystyrene foam. Has many other uses including swimming.