aka Young Sherlock Holmes and the Pyramid of Fear, Pyramid of Fear
If this was released today it would probably be called something like 'Sherlock Holmes: Origins' or 'Sherlock Begins' seeing as it tells of the first meeting between Holmes and Watson at boarding school, their first case, and the origins of Holmes' look and personality. There's even a post-credit scene to set up a sequel (that was unfortunately never made).
Putting all that aside though, this holds up as a fun and family-friendly adventure/mystery and is exactly what you'd want/expect from a film directed by Barry Levinson and written by Chris Columbus.
However this remains most memorable for it's physical and computer generated effects - it was the first film to feature a fully CG photorealistic animated character.