The 18-55mm kit lens is the one you want... most likely. Canon also developed a 15-45mm as a replacement kit lens to the 18-55, which is: smaller, wider, cheaper, and slower on the ‘far end’. If you had no lenses, and just the body, I might suggest the 15-45mm instead since it’s only ~$100 used. But, you’ve got the 18-55mm... so it’s not worth getting the 15-45mm since they’re “effectively duplicates”.
Most of your “nature” shot examples are probably closer to what most photographers would call “landscape”. Your kit lens’ 18mm (~29mm view on a 35mm film camera) is wide enough to learn the lessons of how to frame and shoot wide-angle shots... which are not as easy as they might seem. That lens’ 55mm will be good for some simple “portrait” photos, although you’ll have to work hard because the f/5.6 minimum aperture won’t provide a lot of subject separation from its background (but it will be more than the 15-45’s).
The “pancake” is most likely the EF-M 22mm f/2 STM. Some reasons your dad probably liked it was that’s half the size/weight of the 18-55mm kit lens, and is 2/3 of a stop “faster” than the kit lens’ fastest speed. Also, the fixed 22mm focal length is about the same as a 35mm lens on a film camera... a view which was used extensively/exclusively by most news photographers over the last 60-odd years.... in addition to being the ‘stock’ lens on most ‘simple’ film cameras. So, many people are familiar with how to frame shots with that lens, would like the lighter weight, and faster speed.
Should you get one? Meh. Not right now. I agree with codrus, go out and shoot a lot.
The “nifty-50” everyone’s talking about would be a ~35mm focal length lens on the EF-M. And Canon has a 32mm f/1.4, but it’s $500. Also, I wouldn’t worry about everyone saying you should get an 80D or other APS-C DSLR.
Instead, if you’re up for the challenge, I suggest you consider getting a good, but “dumb” FD to EF-M lens adapter. This will allow you to mount up Canon’s old manual focus, manual aperture lenses to your EF-M. Now, doing so will require you to “skill up” and be comfortable choosing the right aperture, and manually focusing. However, you’ll be able to pick up several lenses for dirt cheap off keh / amazon / eBay.
tl:dr
your 18-55 will do just fine for the trip. Enjoy, and don’t drop it in the water.
when you get back, do some research... maybe “skill up” a bit, and unlock shooting with cheap manual vintage lenses.