The only morale event that matters is shipping something you’re proud of
No amount of brainstorming sessions or getting to know each other or team dinners is going to meaningfully and durably improve your team morale. They all help of course. Getting to know the people you work with on personal levels is important (for those who want that).
But none of these things can counter being on a team that doesn't believe in what they're shipping. Or one that doesn't have pride in what they're shipping. Morale events might make that day, week, maybe even quarter better, but they're not getting at the core issue.
The one thing guaranteed to improve morale is to ship something you're proud of.
Ship something you know actually helps people. Ship something that's the result of solving really hard problems you weren't sure you could. Ship something that shows the best of what you got. Ship something full of craft and care.
Ship something you're proud of.