Slay the Spire is a deck mainstay for me. A good run is 40 minutes to an hour.

Stardew valley can get like 5-6 hours of battery easy on the steam deck

How about SNES or GBA emulation? For what it’s worth, many flights now have a power outlet between the seats

Chained Echoes. 16 bit RPG with great art and gameplay, but it's not hard on the battery and it has modern day QOL features.

Emulating Gamecube games is pretty low-spec if you compile the shaders beforehand. Battery life nets 2-3 hours on flights for me (Steam Deck). I have nearly 2000 hours of Animal Crossing GC combined, pls don't judge me 😅.

Tunic, hades, dead cells, some old rpg like fallout nev vegas or fable

Check out Celeste if you haven’t already. For me it’s the perfect 2D platformer and works really well on the deck. Think I clocked around 100 hours on it just on it alone!

Stardew Valley would be my pick.

Pizza Tower is fun.

Baba is You can last for over 6 and a half hours - the longest battery life I've ever seen on my Deck. You just need to enable the minimum TDP limit.

I also believe Brotato lasts a lot, Pixross too.

Most planes nowadays have a charging port where you can plug in your phone for the duration of the flight. Therefore, I recommend getting an emulator on your phone and playing games on it. Usually if I want something casual, I get an NDS emulator and play Pokemon Platinum or HGSS. Other recommendations are FFIV, Etrian Odyssey III, Shin Megami Tensei, DQ4, some of these are ports but they run well on DS emulator.

Hades was practically made for Steam Deck

Play some factorio. You'll play it forever.

The factory must grow

Dead cells

If you prefer some story-heavy games, you can try Eastward. It has gorgeous art style and controls and UI are a great fit for handheld devices such as Deck and Switch. It takes ~15% battery a hour and can be completed in 10 to 15 hours.

If you're fine with longer games which will take 45-70 hours total, try some 2D JRPGs like Trails in the Sky FC (which also has nice touch input in menus!), Trails from Zero and Chained Echoes. All tree aren't battery-heavy at all.

Bastion, Celeste, Hollow Knight, and Tunic are all great. I also really like emulation. I've been playing Tears of the Kingdom more on my Steam Deck than on my switch!

Portal!

I don't know how draining Celeste was, but it can't be too much. Most indie games probably work.

Dunno if it's your jam, but I just picked up Dead Cells and it's super light weight power wise. Also, retro emulation is usually pretty good on the battery. Get Emudeck going and some GB or GBA games and have a blast.

Terraria plays really well IMO

Dredge Dredge Dredge!!!! GOTY for me personally. And I'm normally a desktop pc gamer.

Rogue Legacy. Good game, replayability, doesn't drain the batt

Tough to give anything concrete without preferences but I'll toss out a few. Chronicon is a great ARPG that's easy on the battery due to simple graphics (don't let them fool you). Voxelgram is 3d picross, if that happens to be your thing. Dicey Dungeons is a dice based roguelite that's totally full of charm.

If you're flying commercial, you should be able to charge your steam deck while you fly.

I found civ5 was great to play when i when on a 4 hour flight.

Star Traders: Frontiers on Android. Space trading rpg with depth and story. A bit grindy bit there's so many paths to take it can suck up hours (mind you it's difficult and you will die permadeths with many characters, it's mind boggling how many ways there are to succeed, just many more ways to fail too. Great way to spend a five hour flight.

If you're into 2D platformers.
Monster Boy and the Cursed Kingdom.

Cat quest will take you about 5 hours to beat and is light to run. Streets of Rogue is good too.

FTL, Into the Breach, and Slay the Spire are mostly puzzle games -- but since they're roguelikes they have a little depth. Depends on what you mean.

I find them very replayable but I like puzzle games because I have bad hands and game controllers are not an option.

If you haven’t already bought it immediately because of the gut punch of nostalgia then maybe it isn’t for you, but I have to recommend Megaman Battle Network

You could get Idol Showdown. It’s a 100% free fighting game that’s only 200 MB. It might not be the only game you’d want to take along though.

Valheim, Satisfactory, Factorio, Fable, Fallout 3, Fallout: New Vegas, Fallout 4, Super Meat Boy,

I just 100%’d Path of Giants. Very relaxing and cute puzzle game that can kill 2 - 3 hours.

Been loving Gato Roboto, cute goofy mech metroidlike where you play as a cat driving mechs around a metroid style dungeon/base crawl.

I get 4 ish hours emulating the switch version of diablo 3. Easy to sink that much time or more into it

Not sure if it's the type of game you play, but Terraria has a lot more depth than people think, and it (should) be really light on your battery

I like playing through KEMCO games when I'm bored, though they're mostly turn-based JRPG games. My favorite is Asdivine Dios.