Congrats, Random. Your boys sound like the guys in my boyfriend's chapter. I became a sweetheart pretty much the same way you did. And no, James, the boys don't have unresolved or resolved sexual tension regarding me.

I've been dating one of their brothers for over 3 years so I'm thought of as an extension of him more than anything else.
I think it's nice that you want to do something for them. Just keep in mind to keep the "gift" small. Going overboard can make people feel awkward, especially guys. See if you can do something small but fraternity-related. I think keychains were suggested? Sounds like a nice and inexpensive option to me.
ETA: I'd just like to add that I think in many cases, fraternity men respect the women who hang out with them who contribute a great deal to their sorority, as Random does. In many cases I think that's WHY they choose some sorority women as sweethearts. The guys here see that I love and uphold the ideals of my sorority, and since they do the same, they have a great deal of respect for that. When you have that common bond, it makes choosing a woman who you think represents what your group is about (to a certain extent, of course) that much easier.