What have my eyes seen? The faithfulness of God? His care? Specific answers to prayer? Surprising evidences of His presence? Truth in dark places?
"Only give heed to yourself, and keep your soul diligently, so that you do not forget the things which your eyes have seen." PAY ATTENTION. Write things down. Keep a journal. Tell those you do life with. Put a note on the fridge. Just DON'T FORGET! It's no wonder God set up so many feasts and festivals for His people. They were memory aids! Because we forget.
And frankly, remembering is good for the soul.