I recently started a new piece on the piano that's fairly above my skill level, but I've been making progress, albeit very slowly. I'm most struggling with large intervals over a long period of time. I can reach them, but they become pretty uncomfortable after practicing for a while. Afterwards, my hand will continue to feel sore for some time, particularly the pinky region where most of the stretching occurs. I'm aware this is a very common occurrence, so I figure there are likely some sort of cool down exercises to relax the hand. I play the trumpet along with piano, and I often use cool down exercises to relax my face after a tiring rehearsal. This helps the sound quality a lot for when I next play. Trumpet playing doesn't very easily translate to piano playing, but I'm hoping that there exists some piano equivalent. I was unable to find any answers either already on this site or on the Internet, so here I am. What, if any, cool down exercises are there to help relax the hand after an especially strenuous time on the piano?
Before I talk about cool-down exercises-- I'd be wary in your situation as you describe your feelings as "sore" and "uncomfortable". After a good practice session, you may feel a general sense of tiredness, but you should NEVER have pain or even soreness. Robert Schumann invented a device to "stretch" his fingers and ended up with an irreversible hand injury that prevented him from playing piano well for the rest of his life. You should not try to over-stretch your fingers believing that you will get used to it and the soreness will go away.
There are very adept and even professional pianists out there who have a small handspan and can only reach octaves or sevenths and no further. I'm not sure if you have a teacher now, but it may be useful to take a few lessons to learn what you can do to help your playing without having to over-stretch your hands. Since you mention your pinky area, I'm wondering if your arm/hand position is aligned correctly. I have many friends who have small handspans and their teachers are always mindful of coaching them how to accommodate it, and they do play advanced repertoire-- very well, in fact.
For cool-down exercises, I'd recommend something that doesn't require so much "stretching" and in which both hands move independently of each other with minimal pedalling, so if you're at the level where you can play Bach inventions, I'd suggest those.. or any similar Baroque pieces or even early classical sonatas or sonatinas. You can also take your "cool-down" period as a time to practice sightreading easier pieces. Scales are always an option, as well.
A quick search on the internet gave me this: http://www.pianocareer.com/piano-health/piano-wrist-arm-shoulder-warm-up/ Although they are warm up exercises, they might help prevent this sort of thing. Also, splitting up your practice sessions into 10-15 min sections with 5 min "breaks" (where you get up or play something less strenuous) may help.