Oh hello SO: I wonder if you have any good ideas for fun group desserts for a family-reunion-type setting. Here are the ideas our group has done so far:
• Sundae bar: ice creams, toppings
• Cookie bar: many different kinds of cookies
• Candy bar: many different kinds of candy
• S’mores
The candy bar was THE BEST/FUNNEST when we had a ton of little kids who went WILD over all the options. Now that everyone is older, the idea seems to have lost its shine. Sundae bar feels infinitely appealing, and we always do it. S’mores is another always-do. Cookie bar (Milanos! Oreos! wafer cookies! maple-leaf cookies! Keebler fudge-stripes! fudge-dipped Nutter Butters!) was fun once and will probably be fun again, but was not quite as fun as hoped.
In general it seems we favor highly-customizable displays, where everyone can take exactly what they want from a wide selection. But that is not a necessary preference to comply with, as we collect ideas.
It helps if the idea is make/buy-ahead-able and transportable, and/or doesn’t take up too much fridge/freezer space. We are in a rental house and we don’t usually know the fridge/freezer space ahead of time; and I don’t think any of us want to spend much of our precious vacation time making food.
We are talking SMALLISH reunion in this case—like 12-16 people. But feel free to suggest for smaller or larger groups: these comment sections are for ALL of us.

If you are just looking for something that works as well as ice cream sundaes, but is not ice cream sundaes, what about pudding? I think pudding gets forgotten about, but it is delicious. And probably better for adding fruit to than ice cream is. And those that are so inclined can make pretty parfait looking things, which is harder to do with ice cream.
We like doing a bake-off. Get 4 brownie mixes and some mix ins and baking tins. On teams, the contestants add fruit, jam, marshmallows, chocolate or other candy, nuts, sprinkles, etc. whatever sounds fun. Then everyone is a judge, and ranks the combos, but also, everyone wins because it’s delicious!
This has been great for keeping kids occupied too.
You could do bar cookies (everyone bring their own. They are usually freeze able so you can make them ahead and bring them with you). But I’d probably just swing back on the hits and just change it up. Instead of sundaes do DIY ice cream sandwiches. Instead of cookies do mini pastries. OR look into services at your destination and everyone chip in a have a catered dessert table!
Ooo. I like pudding idea.
I was going to say buy 3 dozen plain/glazed doughnuts and let people put toppings on them.
Or a sweet + salty where half the group brings sweet (chocolate, marshmellow, etc) and half brings salty items (pretzels, pringles, popcorn, nuts) and then you can mix and match.
Hm. Crepes with different fillings.
Various pies?