Give them Crimea in exchange for an agreement that Ukraine be allowed to apply to join NATO.
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.
There is no "and then" with NATO member sovereign territory.
Ukraine is only being invaded by Russia because it isn't a NATO member.
Russia was willing to invade Ukraine, for some land. Russia isn't willing to invade 28 European countries + US + Canada, for some land.
Where did you even get this idea? Russia is the largest country in the entire world, encompassing 1/8th of the inhabitable land on Earth ALONE. Why would they get in a fight with the entire world to get a few more hundred squared kilometers of dirt? That doesn't make the remotest sense.