I have a couple of flexible collapsible cooler bags that I use. I put them in either my front basket (mounted on a removable Steco rack) or rear crate (mounted on rear rack). I mostly eat at a café but occasionally will take something along to have a picnic instead.

What? Varies. Sometimes leftovers, sometimes sandwiches. For just me I'll usually take along either a piccolo or half bottle of wine or a full bottle for two of us.