I agree with
@Skillmister having keys to someone else's property would make me twitchy. Just seems such an odd thing to do. If I mentioned this to friends they would think I have lost the plot.
Maximum I would do - use the freedom to plan, measure, obtain quotes, the temptation to clean, tidy the garden would be strong - but then if it all went south, worst thing is you have wasted time and effort. I am not sure I would stretch to this in all honesty, likely use the time to plan and thats it.
I would not go as far as decorating or make any changes (e.g. hack down a tree, decorate) as I would imagine, this could land you in trouble? Would I move in - No chance. Wouldn't be able to sleep also, you get settled and something happens - you have to move out. That would be awful.
Save or minimise the pain, so if you do exhaust some time and energy into the house before completion, you do it with an open mind, at worst - you lose out on effort, and only self to blame.