Solid advice is: don't plan a surprise party.

Ask them. Different people have different traditions and preferences.

Do your family members fast? They will want to eat right away and if they are like lots of people, have a dairy meal - lox, cream cheese, bagels, smoked fish, egg casserole, noodle kugel, salads, fruit and lots of desserts.

Do they go to synagogue on YK? How long are they there? Do they stay all day or are just there for a few hours? If they are there all day, they will likely be exhausted. As several pp’s said, can you celebrate the birthday on Sunday? If the birthday person isn’t Jewish, they may not appreciate a dairy meal.

Sephardic Jews often break fast with soups (premade of course) and cold tortilla (omelet) that can be made the day before.

The other reason for bagels, fwiw, though it probably doesn't apply to a lot of us anymore (I don't follow this particular rule), is that you're not allowed to cook or heat food during Yom Kippur itself. You can slice bagels pretty fast after the holiday ends, and a cold fish platter or whatever is ready to eat immediately.

OP, seems like the consensus here is also the advice I'd have given before everyone else weighed in, which is that if your Jewish family members will be fasting, the evening of Oct. 12 is probably not an ideal time for this meal. It's considerate of you to try to find the answer in advance and to potentially work around the holiday!

I thought one reason for bagels was that it was easy and quick. Fasting is tiring, and people probably don't want to spend hours on their feet in a hot kitchen while hungry and more importantly thirsty. So, having something tasty, and quick seems like an obvious choice.

Had a friend in college whose family tradition was to break the fast with a shot of whiskey, which we tried one year — definitely glad it's not my family's tradition.

FWIW, I think bagels are traditional only for Ashkenazi Jews, but the rationale is usually (a) it's a Jewish-seeming food that's (b) associated with breakfast, appropriately for the first meal of the day even if it's after sundown and (c) as PPs noted, easy to prepare in advance and quick to serve.

I'd do it on the Sunday