Duplicate database and keep it up to date - Replication or SSIS?
I would just backup/restore. This accomplishes the same thing, validates your recovery story, and will easily handle schema/object changes as well - without really putting any undue strain on the existing database (you're already backing it up, right?).  If it were on a different server (or if you ever split them up), I might suggest log shipping as opposed to mirroring or replication, that way you can fiddle with how long between log backups, how long to delay restores, what kind of maintenance window to use, etc. Of course even on a different server a very simple backup/restore will work fine.