The Resurrection of Devan Shell: Episode 1 - I know this will be probably too hard for newcomers but there isn't really anything better out there singleplayer-wise. If they want to showcase something, why not the best? I think it's better to start with the 1st episode though, even when it's not the best out of the pack, as RoDS is quite story heavy.

Ozymandius - Not sure if it's not included in jj2+ by standard, but if not then definitely. This is a showcase of jj2+ and angelscript capabilities, so it shouldn't miss.

Dreamscape - This is my personal favorite, as it introduced me to the feelings of dread I haven't thought were possible while playing jj2. Basically what it does, it takes your expectations for classic jazz gameplay and throws them out of the window, hitting you with quite mindf***ing leveldesigns and story ideas.

The Lost World - It has completely original tileset and tries to do things differently by using more realistic level design and story. It's also accessible for newcomers as it's quite easy and has a quick pacing.

Tileset Extractor - As far as JCSing goes, this tool is pretty much essential for everybody if they want to do any changes in already existing tilesets.