I taught myself to ride with no hands a few months ago. It took a week or two of making myself do it. It's a worthwhile skill to learn. Removing arm warmers, adjusting helmet, stretching, eating are all easier to do, and you don't need to stop moving...

You can scrunch your shoulders while riding. Just lower your breastbone towards the stem and do it...

I used to try and only spin up hills. I find now that I climb best around 70-72rpm, and I stand for most stuff above 10%, but if it goes on for a while I will alternate seated/standing.

I found that I can go quicker at the same HR while standing. It's not the conventional wisdom, but I think I'm still at a point where my glutes/hamstrings/arms are much stronger than my quads. I'm also heavier 180ish, and I just feel like it's easier to get power to the pedals if I'm standing.