Scalars on the stack are "automatic" variables. There is no explicit memory management needed by the programmer. They are pretty safe, as long as you don't do stupid C casting tricks.
The "certain things" you refer to includes using *any* pointer or array value in any way, including allocating, freeing, indexing and dereferencing. That essentially means any operations on items other than the automatic scalars I already covered. (For completeness, we can also consider "plain old data" structs lacking embedded pointers or arrays to be as safe as scalars, and globally allocated scalars and POD structs to be relatively safe, even if generally not good practice.)