The current version of Sketch-Up lets you try it free for a period, then asks for money.
But there are older, totally free versions out there, available for download. Grab one of them, download it, and then ignore it every time it says, “Newer versions are available.”
Here’s a place to start.
More in line with what the OP actually wants…I don’t know of any easier-to-use 3D toy than Sketch-Up. I’ve downloaded trial versions of a few others, and didn’t care for them. Sketch-Up takes a while to get used to, and it has some quirks and bad habits.
(Say you want to move one corner of a cube, to make a pointy blocky thing. You have to hit “Escape” when you’ve got it where you want it, otherwise it will continue to follow your mouse, and you’ll have super-distorted results you didn’t want. Same with a lot of the drawing tools: you end up having to hit “Esc” quite a lot, to avoid “chaining” results.)
(Moving objects around in order to join them to other objects is also finicky. You get best results by moving them in one dimension, swiveling the view and moving them in another dimension, getting them close to where you want them, then zooming in and repeating. Very fussy. Objects “teleport” around, popping to locations you didn’t intend for them to go.)
The learning curve is steep and harsh, but, if you can stick with it, you can get some jolly nice results. Sad as it is to have to say, most other products are worse.