I have a USD of a boat, with two different options for masts in it. I want to create a variant for each. I thought maybe I could run an initial prune node (using deactivate) to get rid of one of them (so this could be the default). And then create a branch with using configure primitive nodes to turn off the default one and activate the other one, and then feed that into an add variant node.

However, when I try this, and then use set variant, it doesn't do anything. This is my first time actually using Solaris without following a tutorial so it's clear there's some basic USD stuff I don't quite understand yet.

What would be the correct way to go about building variants that are essentially different iterations of different components being activated or not?

If you're intending the boat to become an asset in a larger scene, I recommend setting it up using the Component Builder framework. You can use a pre-existing USD as an input file in the "Component Geometry" node, then in different branches you can activate / deactivate the masts and plug them into component geometry nodes.

If you're new to Solaris but have some Houdini experience I recommend Peter Arcara's tutorials here:
https://www.sidefx.com/tutorials/usd-asset-building-with-solaris/ [www.sidefx.com]