Hi
Traditionally you would expect
Matzah - gluten containing crackers. Non gluten versions very expensive so rarely provided unless absolutely necessary.
Hard boiled egg.
Romaine lettuce
Grape juice or sweet wine (4 cups is the traditional amount)
Charoset - made from crushed nuts and apple. Sometimes made with dates. Tradition has it that this look like mortar
Horseradish / bitter herbs. May not be served if there is romaine lettuce. May be just the jarred stuff.
All fruit and vegetables are kosher on Passover.
However not legumes. Not any of the gluten grains. And for most not rice. Quinoa is likely.
Generally whether it's Passover or not I can cater something for all food needs (and I have some doozies in the family) but it helps to know in advance.
I also help with the food at the school seyder (the demo one they do last day of term before Passover in primary school). There the charoset is literally grated apple, we use celery sticks instead of romain lettuce. And we also serve potato gems/ small potato cakes.
At our family Seder we always have veggie sticks, potato chips, berries as well as the traditional foods. As well as what I've listed above these can include: chopped liver, chicken soup (matzah balls optional), roast chicken/turkey, roast veggies, coconut based cakes and biscuits.
I always suggest people not used to the event or not sure about the food to eat first. That way you can just enjoy the company and not worry about whether you'll enjoy anything.