My watercolor travel kit started with a Cottman field watercolor pan set and a Bockingford watercolor small pad that I could carry in my bike jersey pocket on the roads in the UK (water already in bottle on the bike). Very handy and did everything. Added a pencil with an eraser.
After that started to add to that kit and now it fits into a fanny pack with 6 media, all used on watercolor paper, so sketching and/or painting is fully supported in either case.
Mine now includes the watercolors, gouache, watercolor pencils, ink brush pens, Inktense pencils and I've added Ceracolors (cold water soluble wax paint). If I wanted, wouldn't hurt to add pastels. Anything else would simply be gilding the lily so to speak. Pack includes a water bottle, palette, tribrush washer, pencil sharpener, cotton swabs, paper towels, kneadable eraser and a small knife. Several brushes (use only one mostly) and water brushes too. 4 watercolor pads in different sizes. Yeah, it got a bit heavy of late, but still can be slung over my shoulder or held by its handles or belted on my waist. I take this everywhere and it's always in my trunk too.
What you carry is up to your prefered media, size and working style. All you need is a pencil and paper for starters. That Cottman kit is a great versatile watercolor linchpin and I would advise small pad of 140lb. watercolor paper. But you could start with 3 primaries in pro watercolor (tube or pan), a plastic palette, and the paper and be fine. I don't bemoan the Cottman colors. They are sufficiently vivid when handled right and when I replace or replenish the pans I use the Windsor & Newton pro paints.
Is this vivid enough for you?