Dinner is the food high point of the day and a warming tasty mug of soup to drink while your main course is cooking can be a great way to start winding down, as well as getting some much-needed warmth into your body's core.
Instant soup is the most practical type to use outdoors, but you need to choose with care. Some are honestly not very pleasant at all. It pays to check the ingredient list carefully for preservatives, flavourings, etc, and do a taste test. I'd also suggest avoiding tomato-based soups, as they have a nasty habit of staining plastic mugs.