Croissants are always good as an interesting alternative to toast, and you could lay out different flavour jams. Or you could get mini croissants (so they're not quite as filling) and stuff them with cheese and ham, and then just warm them up.
Have a selection of different cereals available, so people can pick what they like, and chop up fresh fruit (bananas, strawberries etc) for everyone to use as toppings.
You could also offer fruit yoghurts, or bowls of fruit topped with natural yoghurt - it's something a little unusual, and much lighter than carbohydrate-based breakfasts.
Smoothies are also an unusual light breakfast choice - blend yoghurt, milk and fruit.
If you want something cooked, then maybe pancakes or waffles - these are something I never eat for breakfast, so I'd see them as special and christmassy. They're very filling, but everyone could just have one or two each, and you can get a variety of different toppings. You could also offer french toast and scrambled egg with bacon or smoked salmon.
Make sure you have a variety of drinks available too - coffee, tea, a selection of fruit juices, milk and chocolate milk if there are children.
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