Our favourite café has a really small menu but does everything very well and I think that's the key. Keep it simple but do it properly and you'll get people coming back. This is what they do:
Selection of nice sandwiches with crisps and coleslaw or you can get any of the sandwich fillings on a jacket potato with coleslaw (prawn marie rose, egg mayo, coronation chicken, tuna and sweetcorn, chicken and bacon, cheese and pickle)
A salad option
Homemade soup and the option to add half a sandwich or a cheese scone.
Selection of cakes and traybakes.
Some cold drinks, tea including herbal tea, proper coffee.
For children they do a half sandwich or half jacket potato option that I think is brilliant or the only other thing they have is a platter or pitta bread, hummus, crudites and cheese.
If you're doing breakfast then I would serve it until 11.30 before swapping to lunch to keep it easier on the kitchen and just keep it really simple. Toast with butter and jam or marmite, roll with your choice of bacon, sausage, egg, cheese, avocado or any combination of the above or a yoghurt and granola bowl with fruit compote.