TITLE: Comprehensive reference for synthetic euclidean geometry
QUESTION [11 upvotes]: Euclidean geometry is a special case of the theory of Hilbert spaces; but in order to convince small children of basic facts, e.g. that the line segments from each of the vertices of a triangle to the midpoint of the opposite side are concurrent, I've found I need to resort to synthetic arguments.
Do you know of a comprehensive reference for synthetic euclidean geometry?

REPLY [3 votes]: Try Continuous Symmetry by Howe and Barker! 
Howe and Barker take the Erlangen program very seriously and prove Euclidean geometry via a synthetic fashion guided by symmetries. 
A very important book that concretely fleshes the Erlangen Programme which at times appears to be high-brow philosophy.