There are a lot of things that you can do with Home Assistant now that everything is setup. Here are a few basic things to get you started.
Home Assistant uses something called Lovelace to handle the UI. It makes things run fast but it is also easy to edit in real time.
The key to any Smart Home is good Automations. Now we will get into how to create a simple Automation and making things happen.
Honestly there are so many things that can be done with Home Assistant and it really all depends on how far you want to take it. There are many examples of massive UI hacks and elaborate automation sequences, but most of that is done using YAML and Python. I recommend sticking with the web interface if you are just starting out and you will find plenty of things to do.
When you are ready, the main Home Assistant website is a good resource on how to do custom edits, make your own integration app, creating a new card for Lovelace, or even using data templates. Here are a few links to help get you well into hacking your Home Assistant.