Stellaris: Lithoids Species Pack

It's not a beginner friendly species to start with but if you have Utopia DLC as well and choose the Terravores civic to become a devouring swarm that completely turn all the planets into broken worlds that give more minerals when building mining stations around them, while all the pops from the broken world go to the capitol. Also terravore ships get passive regeneration of their hulls and armor so you can basically repair in space without the need to go back to a base.

Its more than just a bunch of portraits, but also fairly minor - worth getting if you want to play as a rock, worth skipping if you don't care

It adds more variety to the game whether you intend to play one of the added races or not. The additional looks of the ships and cities look cool. The additional mechanics introduced for these races can bring some freshness to the gameplay.