I'm one of those who wants as much as possible inside the rucksack where it is safe, dry, quiet, and not going to snag or shift and throw off my balance. Even seeing someone with a rollmat on the outside gives me a twitch! Let alone people with important things like water bottles on the outside. My favoured daysack doesn't even have exterior pockets or a lid.
So, what do I have on the outside of my rucksack? The only thing I'll always have on the outside is a watch, strapped to the strap at my chest. I hate wearing watches, especially with gloves, but look at the time frequently. When doing nav that involves timing, I'll use its stopwatch function, pausing it whenever I stop moving.
If I feel the need to carry them, and aren't actually using them, I might have sticks, axes, shovel and/or skis on the outside.
When scrambling, I might clip some gear to the gear loops on my rucksack shoulder straps as well as on my harness.