Trying to plan for landscaping and hobby shed.

Hello all. I have this space in my backyard that I would like to start planning a hobby shed for. In addition I'd like to build a walkway from my deck, that goes to the hot tub and then the shed itself.

It's the low part of the yard so I think I want to build it on piers instead of at ground level, and I'd be talking out this tree that's struggling anyway.

Is there any free software any of you use to design something like this in 3d? I'd like to play with it that way to iterate quickly, but not sure if there are good free options or not.

Thanks in advance.