What is in the waaaayyyy back of the under-sink cabinet? Pull things out and find out! Replace the things you use and need in clearly labeled containers. Discard what is no longer needed.
One of the best things I bought: under sink mats! They cover the base of the cabinet with a lip to catch any spills or water drips. These saved my cabinet when there was a small pipe leak. Easy to wipe off or rinse off, spills will be contained. And make the most of available storage space under the sink with an adjustable under sink shelf with slats that work around plumbing or two-tier slide out shelves that side to the left and the right of the plumbing.