What is their AFSC? I am not someone that is hot on the challenge coin. The main reason why is if they are going rated they will need to carry their squadron coin. By the time they leave the AF they will have a ton of coins.
Have you thought of downloading a det picture and framing that? Especially if there is a unique picture, for example, at our DS unit they have a seniors graduation military ball. Many parents attend the ball take a picture of them with the folks. They typically bring out all the 400s and announce their assignments, so you could also take a picture of all of them together. It can be the start of their I love me wall.
One other to think about is some nice cigars with an AF zippo lighter. Smoking cigars still occur in the AF, usually during the squadron outings, formal functions and naming ceremonies.
OBTW DS did get golf balls with the USAF logo from my Mom and she also ball bought him the monogrammer for him with his initials to use in the future on new golf balls. It was one of his favorite gifts and he still uses it to this day.
As for the flag, we did it for our DS. if you go down that path check with his folks to see if they are going to do it.