The space needle very slowly spins if you're in the top, if I recall correctly, so walking around in it is a bit funky.
The building formerly known as sears tower, much higher than the space needle, part of it is glass bottomed, can look straight down from it.
The arch it's all just an odd angle and a bit cramped at the top I recall
Have not been in the washington monument. For that when we were there you had to get in line or sign up at like 6 am. Too early on vacation with 2 kids