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?