I've got some free time coming up and I want to dig into some different parts of Houdini. I'm lacking in knowledge about sims in general, VOPs and VEX. What would be the best use of my time? I'm thinking things like FEM, L-Systems, POP Grains, etc. aren't very frequently used and not worth spending time on? Maybe focus on pyro, vellum and RBD/destruction? I don't have any specific projects in mind, just want to broaden by base of knowledge. Thanks!

I've recently had a lot of fun playing with the new fancy features:
Pyro minimal solver -
https://www.sidefx.com/tutorials/minimal-opencl-solver-part-01-explosion/ [www.sidefx.com]
RBD guided simulation -
https://www.sidefx.com/docs/houdini/destruction/guidedsims.html [www.sidefx.com]
Vellum brush along with old but still relevant vellum clothing tutorial -
https://www.sidefx.com/tutorials/november-thug-vellum-with-sara-rascon/ [www.sidefx.com]

If you want to get serious about vops and vex fundamentals I'd recommend:
Vex for algorithmic design
https://www.youtube.com/watch?v=V5i6KM_-8X0&list=PLzRzqTjuGIDhiXsP0hN3qBxAZ6lkVfGDI [www.youtube.com]
Hipflask courses for VOPs, parameters, curves logic:
https://www.hipflask.how/courses [www.hipflask.how]

Nice, thanks a lot! That VEX series looks especially interesting.

Yes, the Junichiro Horikawa vex series is really very good, and think having that solid understanding of VEX is a big help. Kind of feels like a bit of vex can get you out trouble in so many areas when you might otherwise get blocked.

These are great too:
https://www.youtube.com/playlist?list=PL2ErPsQ-O7_cOC87ja4ATTinrIJz9zGJE [www.youtube.com]

Cool, studying VEX will be my new project. I can do a few small things with it now, but I have a lot to learn.

Good choice Also I found that knowledge of VEX then will help you to feel yourself more confident in VOPs, as well as, to make a distinction between VOPs nodes which are directly replicating VEX functions and which are unique or high-level/extended wrappers for them.

I would suggest only diving into VEX if you want to solve specific problems.
Otherwise you will never retain the syntax. Matt's tokeru VEX stuff is more all round type
of things you will use daily, with only a bit of stuff that is one off.

Yunichiro VEX stuff is higher order, more esoteric. I'd recommend covering all of Matt's stuff
before venturing into that area,and more only if that type of thing really interests you.

pyro, RBD, vellum are all good ones, they can get very deep and very fiddly, so take it slow!

I think making sure you have an overall solid base of "how" houdini works, attributes and their roles
in controlling things is a good spot to work towards. To that end, again, Matt's tokeru stuff will mostly
be working on spheres, pig heads, grids, and other simple geo, it will give you a solid base with zero distractions
in which to learn.