Give them Sudeten in exchange for promise that they will not attack Poland.

This tactic worked well for 8 years in the case of Ukraine and was heavily (really heavily) promoted by François Hollande and Angela Merkel.

IMHO, it's good to stick to a tactic with proven track record. The main idea is to feed the monster with virgins. Give them Crimea, then Donbas, then Belarus, then South Ukraine, then Eastern Ukraine, then Central Ukraine, then Western Ukraine, then Georgia, then Finland, etc., etc., etc. With a few years between each scarification, this can keep the monster satiated for a long time, so it will be a problem of your children, while you keep a comfortable style of life. For bang per $, this is the cheapest and safest option for most Europeans and Americans.