Have you ever gone on a callback before? That sort of thing isn't really necessary and also could lock you into one practice group. Firms don't want to hire you if they can't give you work in an area you seem committed to (and firm websites are generally pretty misleading about the scope of the work in particular offices). If commercial real estate isn't hiring (and real estate finance is down generally), you could wind up without a summer offer from the firm or, worse, you could wind up no-offered at the end of the summer.
I'd go general and just say "One thing I'd like to try out over the summer is some commercial real estate work."
I've been on several callbacks. In discussions with this firm, I've expressed my interest in commercial real estate law, which for them is still booming, and they have been marketing that fact to me. Your point is well taken about getting locked into a practice group, or getting no offered. I agree with your statement about firm websites being misleading.