My kid asked to sit in on classes in her intended major. Of course, that only works if your child knows what he/she wants to study and many times that will change. Some schools will let a prospective do overnights. Carnegie Mellon has specific Sleeping Bag weekends throughout the year that allow prospectives to get the dorm experience.
I would definitely recommend doing an admissions interview as a junior especially if you are visiting campuses that you can't revisit easily. Have your kid dress conservatively as he/she would for a job interview. My kid always wore a business-length skirt and blazer for her college interviews. Teens don't wear hose much but she did for interviews. If your kid is applying to art schools or alternative, laidback colleges such as Reed or Hampshire, the conservative look is not really appropriate. I interview prospectives for Carnegie Mellon which is a university comprised of very distinct schools. The boys tend to wear suits or a jacket and khakis, the girls will wear a skirt or good dress pants. Even the kids applying to the College of Fine Arts look pulled together and ready for an interview.
My child also brought a resume to leave with the interviewer. No matter what, follow up the interview with a handwritten thank you note (even for the laidback schools)!!!!
If your child plays a sport or instrument and wants to continue in college, set up a meeting with the coach or music department. Even for D3 schools that aren't allowed to give out athletic scholarships, athletic prowess can help in the admissions process.
Obviously your high school guidance and college career departments have statistics on admission rates. Ask if they use the computer modeling program called Naviance so you can see the scattergrams for college/university decisions for the specific applicants from your high school.
I know some GCers don't like these sites but I believe that
http://talk.collegeconfidential.com and
http://admissionsadvice.com are both good sources of information in the college search process. CC gets a lot more traffic than AA. Many of the posters are from the northeast and are HYP-bound (or want to be). Posters will post their kids' stats to get suggestions of potential schools. You'll see what I mean after reading it for awhile. The parents' forum is the busiest and you will get the quickest and most responses there.
To get to the message boards of AA, click on the left hand side of the screen. It's a kinder, gentler CC. Both AA and CC are run by private college consulting firms but use of the forums is free for both.
Feel free to PM me. I love the college search process which is why I am still so involved although our next kiddo is only in 8th grade.