I think taking them on a little field trip is alright as long as you keep a real close eye on them and follow a few precautions.
Like, dont take them into direct sunlight - shady with a few sun "patches/spot" is ok. The temps are fine anywhere 78 F and 95 F, but consider the level of humidity out that day (too much is not good for their respiratory system). And think about where you will let them go (they should not be left on the ground for too long, or be aloud to wander on their own). Remember there are other animals and even some bugs that would do great harm to your leo if they had the chance. Or if your leo gets curious and chews on some plants or snaps up a few foreign bugs it could be a really bad day.
So, if you want a little fresh air and think your leo would enjoy it too.. then think ahead and be prepared - and never let your eye off it for a moment!
Good luck on your decision!