Are there any freeware apps out there for 3-d construction-type things?

I'm planning on building a kind of strength/agility course (using things like the things on ninja-warrior and some less difficult things) and I'm very bad at drawing this kind of thing but I want to pre-plan what I will need for it and the proportions for it and everything. Basicly all I need is a programm that I can make basic shapes like rectangles and cylinders etc. and set them to the proportions I want and put them where I want, then view them in a 3-D view so I can see all around it. I.E. if I wanted to make monkeybars I would set some semi-thick, long cylinders uprights, then ones resting on top of those, then ones going between the top ones at even intervals, and then I would want to be able to go through them and look around them and everything in the app to find out if it looks right and everything. Thanks in advanced