I think в should be used where it could potentially be replaced by вовнутрь (into), i.e. I think in all cases where it is used we are talking about some sort of an area or building that we will be entering.

К is used when you can ask a question к кому/чему?
Давай подойдем к пруду. Let's approach a pond. Давай подойдем к чему? К пруду.
Пойдем в гости к Лене. Пойдем в гости к кому? К Лене.