For fun, I created a tiny subset of the Scheme R7RS Small specification.
The draft version is at:
(PDFs are available in the releases section)
The tiny subset removes non-functional features, continuations, non-integers, and other features that complicate the semantics or implementation.
It leaves a small subset where the grammar can be described in about a page, and the semantics can be described in another page. The whole thing is 17 pages.
The draft is a superset of the Scheme subset used in the book "The Little Schemer".
Because of the simplifications, the type of an expression is:
Environment -> Expressed Value
Environment -> Dynamic Points -> Expression Continuations -> Command Continuations
which results in a much simpler semantics.
(I also created an implementation of scheme that more or less follows the specification, but that is another story.)