There is usually some ebony and sometimes some ivory in the keys which can be worth saving. The ivory is usually pretty thin if its there but could work for inlay pieces.
Some of the sound boards are OLD spruce which can be worth it if you want that sort of material for projects (mandolin tops, etc..) although there are a lot of holes in them to so I'm not sure how generally useful they are.
If its old enough there may be some mahogany or other bits in in the legs and bracing that would be interesting, but that means somewhere pre-mid-1800s (made sense in my head I swear).
A lot of the other stuff is veneer on piano's which can be of dubious cost/benefit to recover.
If you do do it be EXCEEDINGLY careful taking the strings off, the amount of tension in those is remarkable. Get a tuning wrench and loosen them a bit at a time in series to avoid having major bits go SPRONG on you.