Haven't done a great deal on day 1 but there was one thing I had been wanting to do since I got myself one of these for Christmas...

In Unity, you need to build for Linux (which you can do on Windows, macOS, or of course Linux) with Auto Graphics API set to off, having only the Vulkan API in the list. You can get the resulting build on the Steam Deck fairly quickly and easily by using the SteamOS Devkit Client, which you can download on Steam. For best results, test your game in the editor at 1280x800. I haven't looked yet at how to get the more fancier controls working, but the standard controller stuff works as expected. One could always use a controller layout, but it would be fun to have some kind of native support for the trackpads and the gyro at least.