Sometimes, the opportunity arises for a spot of painting outdoors and the weather even co-operates, but by the time I've thought about what to take out of my normal painting equipment, half my painting time has gone. To get around this, I keep a light-weight rucksack ready to go with just 10 items already packed:

watercolour travel palette, with colour in pans and space to mix
brush, squirrel hair round, size 12 (an elastic band holds this against my palette)
black ink, fine tip felt pen, for sketching (optional watercolour markers)
A4 watercolour pad or block
a few sheets of kitchen paper towel
a refilled, litre plastic bottle of water (for painting and drinking)
a recycled plastic yogurt pot, (or collapsible water pot)
small spray water bottle
sunhat
small towel, to sit on

My 10 bare essentials, for painting 'en plain air'.

With these bare essentials ready to grab and go when the chance appears, I no longer waste any time collecting what I need, when I could be painting instead!