I saw in the name of the *Or Zaru'a* and others that one does not make a *b'racha* before writing a *sefer Torah* even though it is a positive *mitzva* with an action associated because (among other reasons) one can be reasonably certain that he will not perform the *mitzva* exactly correctly since it is such a hard thing to do. However, one still says "*Shehechiyanu*" upon performance of this great *mitzva*. This led me to the question. . . 

Performance of what other *mitzvos* warrants only the (time-based but not intrinsic) *birkas "Shehechiyanu"* but no *birkas hamitzva*?