Some guns have adapters that work rather well. Surefire makes a couple for a very limited selection of pistols ( 1991, Sig, HK ) and someone else makes one for Beretta 92/96. As far as I know, and I did check around online and all my parts catalogs, there is no such adapter available for a 5906 smith.
If you feel you need at least some kind of mounted accessory, the best you might be able to do with readily available parts would be one of those trigger guard mounted lasers. They can be found cheaply on ebay, but personally I would HIGHLY doubt their quality. Although advertised for use on real firearms by unknowing or uncaring sellers, I believe these were designed for airsoft use.