So, if I were you, I_d wear a sundress with a wide brimmed hat. You_ll want to be comfy. Watch weather.com (or your choice of weather sites) for updates and take an umbrella just in case.
How long you_ll have with your boyfriend depends on how quickly his drill SGTs release them. Does he have his orders for AIT yet? And will he stay at Wood? You_ll probably have time together. His orders will indicate when he has to report to AIT.
I_d suggest getting away from post _ heading to Springfield. There will be more hotel choices there _ more stuff to do.
Edited to add: make sure you have comfy shoes for the graduation ceremony. There may not be enough seating for all the family members present.
Edited again: Check out the Fort Leonard Wood website. There are specific rules for those allowed to go off post.
http://www.wood.army.mil/