Itch and Kenney have good ones:
https://itch.io/game-assets/free
https://kenney.nl/assets (all CC0)
Synty also has a nice placeholder pack for $7. The post-it notes are kind of adorable:
https://syntystore.com/products/polygon-prototype-pack
I don't think most of these are made for Godot, so you may have to mess around with import settings or set up tilesets/materials yourself
I don't know how materials work in Asset Forge, but they have a guide on their site for exporting models to animate with Mixamo: https://kenney.nl/knowledge-base/asset-forge/rigging-a-character-using-mixamo. You could also animate things like moving platforms or doors in-engine with an AnimationPlayer.
Speaking of Asset Forge, Kenny Shape is a similar thing for quickly throwing assets together. It has a really fast 2D workflow for creating 3D models that reminds me of Doom mapping a little bit. For lo-fi levels, you might also like Crocotile 3D or the combo of TrenchBroom + Qodot. Crocotile is great for repurposing 2D pixel art tilesets from itch or OpenGameArt into 3D assets, and Trenchbroom/Qodot is a more fully featured level editor I've seen people work crazy fast in.