This is a stupid question and I know this has to have been covered here--I just can't seem to find it. Is there a "standard" non-musical stage version of A Christmas Carol? I guess I'm asking what is the safest choice.

What would you suggest as the easiest musical adaptation?

I don't know about the musical versions ... but my favorite straight version is the adaptation by Israel Horovitz. I believe Dramatists carries it.

There are several straight adaptations of this available at Sam French... most require immense casts, or are frightfully abstract.  There is one public domain straight adaptation as well (the original stage adaptation, afaik), and it suffers from unfortunately awful dialogue, but is otherwise a fairly simple and straightforward adaptation.

We're looking at one called "Jacob Marley's Christmas Carol" for next season... it's the familiar story, told from Marley's point of view.  Four actors play all the roles.  It's a dark-ish comedy.

Also worthwhile is the "Farndale Avenue Housing Estate Townswomen's Guild Dramatic Society's Production of 'A Christmas Carol'" ... A comedy about a terrible production of the aforementioned public domain adaptation.  Did very well for us, and is very funny.

Avoid Israel Horovitz's adaptation like the plague!  The script is terrible!  

Philip Grecian wrote a version and if it's half as good as his adaptation of A Christmas Story then it'd be the best "A Christmas Carol" money can buy