Something I like about all of them, actually, is the way they throw their Dynamax/Gigantamax Poké Balls. They each do it as such (I have the Shield version; I haven't observed the Sword ones yet:
Milo - He pets the ball a couple of times before throwing it, showing how much he cares for his Pokémon
Nessa - Swirls the ball for a moment before giving it a toss with a lot of spin
Kabu - Holds the ball low with his eyes closed, as if he's focusing deeply; his eyes briefly ignite as he throws it
Allister - Lets the ball grow in both hands (all others do it one-handed), then throws it back while swinging around; all his motions indicate the ball is quite heavy for him
Opal - Turns around, then does a gentle double-handed underhand throw, as if she's throwing a bouquet or releasing a bird
Melony - Cuddles the ball against her right cheek before throwing it
Piers - N/A
Raihan - Carries the ball on his shoulder like a basketball, then has Rotom take a selfie before he throws it
In particular, the way Melony treats the Poké Ball suggests treats her Pokémon in a "Mister Muffykins" way, which I believe would be a first for a Gym Leader (and an unexpectedly difficult one for many people).
The floors of these stadiums might also be a weird thing to be interested in, but each of them has a texture corresponding with their types. Below is a little bit of each of them as found in the Shield version:
Turffield Stadium: Smoothly cut painted grass with light and dark stripes so as to resemble a soccer/football/rugby field
Hulbury Stadium: Clear acrylic, with lines painted so as to resemble a swimming pool
Motostoke Stadium: Cobblestone embedded in mortar, with a thick coat of deep red paint
Stow-on-Side Stadium: Worn down concrete or fiberglass; the markings on the floor also have a worn look
Ballonlea Stadium: Clean, fresh carpet
Circhester Stadium: Slick-looking ice surfaced via Zamboni, like an ice hockey rink
Spikemuth Gym: Bolted cement, with lights shining on it like the dance floor that it is
Hammerlocke Stadium: Stone tiles arranged to resemble reptilian scales