If the frittata is the main and cupcakes are the dessert you need something to go with the frittata so I would say two salads. Maybe a potato which everyone usually like and either a grain type or a pasta.
The roasted veg salad that someone else mentioned would be good and easy too.
Or if you want you don't mind working with filo people always love spanakopeta. But if the frittata has spinach...might not be good idea.
I would do a lighter salad to go with those. A few of my favorite are this really simple french grated carrot salad (i often serve ontop of a platter of mixed spinach and arugala)
I loved this mix of avocado and grapefruit, just use what fresh greens you find instead of watercress if that's not around:
This side dish of leeks is very mild and goes well with eggs:
I have a few old faithfuls for brunches.
1. Smoked salmon with bagels or a hearty sliced white bread and either cream cheese or butter and a pesto drizzle.
2. Very fresh croissants, either from a killer bakery or Trader Joe's freezer version. Maybe a mix of plain and chocolate.
3. A bottle each of Prosecco and Perrier and some peach nectar.
Maybe some bruschetta or canapes.. like beet and goat cheese with candied walnuts or a salad bar?
Do 3 bases.. one spinach, one arugula, one mixed greens.
then have little bowls with toppings, fresh apples, bacon bits, avocado chunks, pine nuts, walnuts, beets. something like that?
bacon wrapped dates
puff pastry with spinach and cheese