Hiya, I agree sandwiches are good as is Fruit salad. If you feel confident delivering an activty that looks at nutrition there shouldn't be any reason for the children or anyone to consider it below them
Festive fruit salad - let the children peel and segment their own satsuma/oranges or clementines, add grapes - halved or whole and a few dried cranberries.
Walnuts & hazelnuts would be nice to include but you'd need to check whole school policy for allergy sufferers and what budget you have. You could also include a selection of orange segments that have been sprinkled with a christmas associated spice - cinnamon & nutmeg maybe and let everyone sample a piece. If looking at starch/carbohydrates/proteins/fats/sugars & fibre serve with a
small coin sized spiced cookie ..
Use small individual cups and if possible, ask if you can plan for a little more than 10 minutes.
Hth have fun with it
xx