One option (that has worked out well for me) was just to add a memory foam mattress topper. They're very easy to cut down to size as needed with an electric carving knife or a bread knife; the electric knife is slightly faster, but neither is much trouble. If you do get one, you probably will want to let it air out for a spell before sleeping on it, at least if it is like mine was.
The mattress without the topper was hard and lumpy and pokey. With it, it's really comfy.