I usually just start a new generation when the first of the parents dies.

But you can try praising them, buying upgrades for the house or for their career or even just new general stuff for the house. Buying them food always seems to lift their spirits, and you can try having someone make a meal, then make sure they sit down to eat.

I get the sense that buying career upgrade items and home renovation gives them the biggest lift, but they're expensive, so if you don't have the coins for that, try some new groceries and a new toy for the kids.