I've read that
potatoes can be left in the ground, but i believe they are very susceptible to pests and diseases. Esp if it rains when fungal problems may also rear their heads.
They will store well in a cool, dark, dry area, like a cellar, for up to 6 months.
Or you could peel and chip some and freeze them too. Also, look at making some
potato and leek soup or similar and freezing that. This can still be done if they are stored and start to go soft after a few months.
Your onions and garlic tops will dry off and lay over when they're ready to harvest.