Look at them. Arrange them if possible. Even add a backdrop and lighting. Most important of all is to appreciate their beauty
Seriously though, I've never heard of using this part of the cactus. Flowers could possibly contain active substances, but unless you're talking quite a few cactii, it's doubtful actives would be present at high enough levels to extract.
Some of the more experienced cactii people might like to correct or add to this.