This island came out as a Unity Asset quite a few versions ago. But I love it and so have the package here for you as I don’t think you can get it anywhere else. This version was updated in V. 2019.1.
Here’s an old warehouse model of Unity’s that I have used in a lot of tutorials. It’s great as it provides numerous rooms and interesting areas to play test AI and effects. I’ve used it for creating sticky bombs to breadcrumb following AI in my Holistic Development books.
Here’s another great Unity asset from older versions that’s no longer available. Astrodude! I use him in my Endless Runner course and several YouTube videos. Here he is for you as a unity package.
Many, many years ago when I started game development I found this great seamless texture pack online. It was created by Aeonic. Sadly his website and subsequent textures aren’t available anywhere else. But I have them zipped up here for you.