New Prius Owner; What should I do now?

Hello and welcome! Congrats on your new car!

Hmm.. well it all depends on what you prefer and how you want to customise. Here are my suggestions:

Cargo net - useful for holding grocery bags and other things you don't want rolling around the trunk

Trunk Mat - Either a cargo liner (rubber) or cargo mat (carpeted). Keep the original carpet clean and new. There's always the plastic bin below to keep wet/dirty stuff

Rear bumper protector - the rubber one (can use either the one from the Matrix or Corolla). It helps protect the bumper from scratches and because it's rubber, you can rest stuff on the bumper if you need to

All-weather mats - either from Toyota or ExactMats or WeatherTech. They're very useful in keep sand, mud, snow/ice and water off the original carpeting. They're also easier to clean when they get dirty.

Some sort of front end protection - be it a clear film or a vinyl front end mask. My hood and bumper is pitted with rock chips and other crap from the highway (And I don't even go on the highway that often ~10% of the time).