Grave Rubbings: Lightweight paper, masking tape, black crayon, a cardboard wrapping paper tube to roll up and store the work, a lightweight dustpan brush to clean the surface. Cover the entire front with paper and wrap it around and tape it. Use the flattest surface of the crayon to make light even strokes. Step back and see if you need to go over any variations in the tone of it so it's even. Clean up everything you used and never leave litter behind. Roll it up and put it in the cardboard tube.
Cemetery Tours: A lot of historic cemeteries offer tours. Why would you do this? Because they do a wonderful job. Many of them wear period costumes, take you around to the famous or infamous graves and learn a lot about the town.
Inventory: Some state's cemetery associations list the cemeteries they need inventoried, but you can also check places like this one online. You can go to a cemetery that hasn't been accounted for, take photos, write down the grave information and submit it. It's a wonderful way to help out.
Ghost Hunt: Most cemeteries don't allow you to stay after dark, but there are those rural ones that are without posting and open access. These can be a good place to give yourself an experience with ghost hunting. Go before dark, get to know the place and take photos of it beforehand because later if you find something on a picture, you might want to compare it and see how it looked in daytime. I've often found colored signs and flowers and vases that with the flash at nighttime looked like something magical. If it's a quiet location, try EVP. Sit down at a grave. I suggest you go to a newly visited one, they generally have more activity. Do not be surprised if you run into shadow people. These are actually very often seen in cemeteries, small human-shaped opaque black figures darting around.
Clean-up: Bring a large trash bag and a handful of flowers. Leave flowers on the graves of those long-forgotten and no longer visited because they are so long gone. Say their name out lout. Now, go around and set vases upright, pick up trash, and leave the place knowing that you attended to those who were unable to do so for themselves.
