Any law school admissions person will highly recommend these LSAC events, even if your undergrad might have one of its own. The majority of schools will be there (this year's attendees
linked here), plus LSAC has workshops on the application process, financial aid, the life of a practicing lawyer, and for diverse applicants. They are all panel-style with panelists usually being more senior Adcoms from various schools (although they won't be talking about their specific schools during the sessions).
As much as some people feel like they can get all the information they need online these days, there's still no substitute for talking to someone in person. If you're interested in the more popular schools, e.g. T14, DC area schools, then prepare to be there for at least half a day. Those tables always have crowds.
People manning the tables will usually be paid staff from the law schools, even if that may not be the admissions dean. Don't think this means the person there is not "important" and therefore not worth talking to, though. If not full-time staff, it will almost always be a recent graduate or current student from that school, and there is a wealth of information you can learn from them. Also, they definitely report back to their offices on people that make strong impressions...both negative AND positive.
I also encourage students to take the opportunity to explore schools that may not be on their lists initially. Insider tip: Those smaller, lesser known, far away schools usually have way less traffic. Except for the Houston Forum, I'm always one of those schools. Standing around talking to no one is BORING. We are usually dying to talk to students and don't really mind (at least I don't) if a student isn't totally serious about applying to my school. You could get some great advice from senior adcoms by stopping by a few of those empty tables.
My biggest piece of advice would be to do some research ahead of time. One of the first inductees into the "AdCom Pet Peeve Hall of Fame" would be applicants who walk up to tables and ask things like "What are your medians?" (or worse "mediums"). The whole point of attending one of these is to get information you can't get from the website.
Many schools give fee waivers at these events, either on the spot or emailed later, but not all schools do this. Nothing wrong with politely asking something like "How would I go about getting an application fee waiver?" after you've asked your other questions.
Hope you have a great time in D.C.!
Dean Perez