You can find some lovely colourful lanterns to hang outside, during the day you won't need any lights in them, as the colours are pretty in themselves. I love the picnic idea, but maybe get some tables and chairs as well, if you have elderly people coming, as it can be hard for them to sit on the ground (getting up can be a hassle). There should be plenty of caterers that would come to the house and cook everything there if you want. Or you can have everyone bring their favourite lunch dish, and you can provide the sweets. What about colourful cupcakes.
Would the garden be suitable for a marque if it were to rain?