I would argue that, whilst certainly not all, there are a great many sacred stones to the Celtic faith. The Lia Fail is perhaps one of the oldest monuments in Ireland attesting to the Celtic faith which is ultimately little more that a large rock. Additionally stones were often used both in henge construction and decorative carvings and although such structures may or may not be linked to the Druids they are almost certainly linked to the Celts.
However I would caution against using ogham as a religious sign as we actually have little evidence of that at all. As Lark said the only surviving ogham stones that we've uncovered so far are those donating ownership and graves. Although In Lebor Ogaim indicates that they were used in a religious setting the book was not written for many many years after the decline of ogham and it's likely the book was guessing at some uses rather than listing them. Ultimately even if used religiously ogham didn't arrive until a few hundred years after records on the druids end so even if used in a religious Celtic setting they're unlikely to be involved with druidry even if related to Celtic Paganism.
As for what we do know the answer is sadly very little, we know a bit about Celtic paganism, although largely from roman, christian and proto-christian accounts which have a degree of unreliability about them due to the fact that the scribing culture inherently dissogreed with the one they were writing on. The other form of perhaps more reliable accounts reaches us in the form of archaeological evidence however this is sadly so thin that it's open to a great many interpretations and again it's difficult to tell if the remains we have today were related to the druids or is simply a Celtic artifact from around the same era.
What exactly being a druid back then meant, you will probably never be able to find out because there are no concrete text or stones or anything else that tells anything definitive. The most concrete thing I can think of would be Stonehenge which was actually just one of two structures of the old beliefs. Stonehenge is thought to be a place for the ancestors and the people of the region would pay their respects there. Of course that is only one theory, and even though it is the most concrete one that I know of, it is still not established fact.
Druids of today revere nature based on the viewpoints held by people of today on what druids were in history. If you want to learn more, I suggest reading the books listed by the other posters and doing some research. I know National Geographic has a pretty interesting documentary on Stonehenge that seems to tie into Druidism a bit, so maybe you could look that up too.
