simpliest way you know of to do this?

so this is what im lookiing to do:

1. say you're trying to figure out what items can fix into a box
2. first you have the limit of the box size
3. then you have to consider that the items are all different shapes

i need something to see/visualise all this basically so that you can see what items would fit

i was thinking about somethign where you can see shapes of the 1) box, and 2) items

and then be able to move them around so you can see what fits

im sure there's other ways to reach the goal but i dunno what those other ways are

out of all softwares, waht do you recommend to do this basic thign?

Fitting items into a container to achieve optimal efficiency is a science into itself.  Ask FedEx, UPS, DHL, etc.  I would think stiff construction paper and a pair of scissors would be the simplest way.  

LibreCAD is strictly 2-D only. Same with the paper and scissors method. If this fits your purpose , in LibreCAD you can easily move around shapes within a rectangle (box). But probably you also will have to consider the height of the items and the box. So apart from the ground view you also have to draw at least one side view. Or use a 3-D cad software, which usually is much more complicated. FreeCAD is the big opensource program for this, but very difficult. You may have a look at SolveSpace, small, but maybe still too difficult.

For me this sounds like nesting for laser or water jet cutting or CNC routing.
Generally this is possible with LibreCAD, but presumably not very comfortable.
Using blocks for the items may be useful and simplify the work flow.

But if nesting is the main reason to use LibreCAD, I'd consider to find another, specialized software for it.
I don't know free nesting software, as I have no application for it, but Google found this:
https://www.google.com/search?client=ubuntu&channel=fs&q=nesting+open+source&ie=utf-8&oe=utf-8

Armin

"Same with the paper and scissors method."

I was thinking more along the lines of building 3D models to scale to find the optimal placement of items into a box.  Just cutting out squares (2D) would obviously not be very efficient.  

someone knows something for 3d

Physical construction seems the way to go - depending on the size of th items perhaps plasticine might help.