Buy seeds, and seal them with little sprinkles of wisdom like fortune cookies in places that would be accessible in the aftermath, the new inheritors of the earth will regard you as a prophet or god when they re-learn the ancient art of farming.
They're designed to survive from late summer to spring, not years or the decades that would be needed for that sort of thing.
Now, there's a ton you can do with seeds - work on optimizing various heirloom varieties of plants to your local climate (as opposed to the "one seed for all places, apply energy to the field until they behave!" approach that is widely used today), and trying to keep some genetic diversity in the seeds you use, but I don't think "random seed capsules" are likely to work over the likely timeframes involved.