I like a combination of brick and stone. The brick can be placed at an angle or stood on a side or end. Cut them in varying lengths to put some zig and zag in an otherwise long, straight line. There are plastic and metal edging materials that come in long strips that you can bend around a bed. The metal edging can be more satisfactory in the long run, but it can be more expensive initially.
Some gardeners prefer plants such as liriope or monkey grass as a border.
And yes, you can mix flowers and vegetables. There are several lettuces, for example, that are ornamental and can be used to border a bed during cooler months or mixed among flowers such as calendulas, nasturtiums and Johnny jump-ups. Broccoli and flowers mix. So do peppers, which will go in the garden in late March and April. You could run beans up a trellis placed in your bed.