That is a lot of contradictory sets of requirements. If it is important to have on the deck then it is going to be trivially searchable online. Something that is niche that others are not really doing is going to be very subjectively interesting or useful. That makes it impossible to recommend anything without violating one of those requirements.

Instead here is some advice for finding project ideas: Look at your own interests/hobbies/things you need to do and start taking note of problems you encounter, grievances or annoyances you have or just things you think could be made/done easier. Out of those you can look at ones that you think a steam deck could help solve and from that you can start to investigate ways to use the steam deck to solve those problems. That is essentially how you find niche and interesting/useful things that are specific to you to work on. It can take time, but the more you think about it and write things down the easier it becomes to find projects to do.

Things that I can just easily stop.

Technically any non-online game will work since you can just put the steam deck to sleep with the tap of the power button when ever you want and resume later on. It takes a couple of seconds to go to sleep and so the only times it is annoying is when you are directly in the middle of some action - which is generally easy to avoid in most games if you know you are coming up to your stop.

Personally I have been playing monster hunter world like this which works quite well - especially since there is quite a bit of less action packed stuff you can do between the main story line.

First off, welcome! You’ll love the steam deck. I don’t have any specific hidden gems but if you’re planning on using this a lot on public transit, look for smaller games to play like Dredge. Anything too intensive will drain your battery a ton faster than a smaller game. Plus it will heat up and fans will be going bbrrrrrrr. May be worth getting a portable power bank if you can’t charge on transit. Also the largest SD card you can get to add to that puppy. Enjoy!

I find KDE connect to be both useful and interesting to use. It’s not really recommendable in and of itself but it makes connecting between your various devices very effective, like file transfer, clipboard sharing, remote control etc.

With regards to casual gaming, I personally don’t have anything to recommend because that’s exactly the kind of game I try to avoid. I like involved games, interesting, with a story, a purpose, requiring thinking and such. But what I can say is that the steam deck is so effective with its sleep functionality that you can very casually play any involved game no matter how much time you have.

It is so easy to be back in the game: I’ve played the witcher 3 a lot (highly recommend it if you have not played it) and it is a very involved game, but I’ve been in and out for even just a few minutes thanks to sleep. It takes one second to be back in the game fighting some enemies and takes a second to pause the game and put the console back to sleep in my backpack. You don’t have to restrict yourself to casual games to benefit from casually playing, with the steam deck.

The main thing IMO is to get some hands on time with it. Figure what you want, what you like, and most importantly what you would like to see improved… and go from there. See if you prefer to use it as a console, or if you want more of a gaming computer and use regularly the desktop. Maybe you’ll want a dock and how you want it will depend on your needs. You get my point :D

Anyway for the laptop see if you like https://nobaraproject.org/download-nobara/ specifically the Nvidia one so it has the drivers pre installed. I use it “everywhere”

Only useful for desktop mode, but KDE Connect is great for syncing clipboards, sharing files, and even remote mouse and keyboard input between a smartphone and the desktop environment.

It’s a game changer compared to using the touchscreen keyboard. Especially with a dock using it as an occasional media PC.