I think no mountain is a great idea! And so is a limited palette! Less colors means less complication, setup, cleanup, dithering around with mixing, etc…
I have a list on “standby” of some paintings that look easier to me that I could paint quickly or with a friend that happens to join me for their first time. My general preferences are:
1) no mountains
2) not too much moving water or “lay of the land” grassy hilly textured areas
3) classic evergreens are good, because even if they aren’t perfect they’re not too hard to get to an “ok” level
4) still water with simple reflections are good too
5) I also like “easier” clouds. For me the fan brush clouds took quite a few paintings to get the hang of, and of course I’m still learning. But the “tapped in” clouds with the big brushes or the “boom/spider arm” clouds with the fan brush are way easier. If the sky looks good, people will be feeling great for most of the rest of the painting!
three simple ones that I like:
Colors of Nature S15E2
Peaceful Haven S13E7
In the Stillness of Morning S26E1
Good luck! Let us know how it goes!