I'm building a platform out of redwood to serve as the floor of a backyard playhouse. The wood comes from 2x4s that have been ripped to 3/4" planks. The temperature will range from 30°F to 115°F with wide swings in humidity. Should I butt the boards up against each other or space them? If I space them, how much?
Since you are suggesting your area has large humidity swings, if it is humid when you lay the wood down, I wouldn't leave gaps. If you do, you will find there will be large gaps between the boards after they do dry out. Since it is humid, the boards will be at their largest expansion. You will never run the risk of them warping/popping out due expansion. If the wood is extremely wet (like pressure treated wood), it will definitely shrink and never be as broad again, especially after a summer gets hot and dry.
Yes, wood moves, you want to space the boards to allow for expansion.
A common way to space boards when doing decking is to use one of your fasteners as a gauge. e.g., a 2" deck screw temporarily slid between the boards as you fasten the boards to the joists. You end up losing some screws that way, but it's the simplest method I've used.
There are fancy schmancy tools to do the same-- e.g.
I doubt you'd drop this one and forget it, hehe.
(Not an endorsement, just an fyi..)