Summer and/or a summer's day is the metaphor that the speaker uses in contrast to the appreciation of his loved one. So, we would need a metaphor that has similar qualities (or disadvantages) to those described here. Summer is short; it only lasts a brief amount of time. Sometimes it is too hot and sometimes it is cloudy. The summer declines over time as it leads to autumn.
The speaker notes that his loved one is more "temperate" than the summer day. In other words, he/she is more restrained and not as erratic (hot, cold, cloudy) as the summer. In the end, his loved one's beauty does not fade, mostly because he has immortalized it in the lines of this sonnet. (The sonnet will last forever.)
So, what is traditionally pleasing (summer) but has the disadvantages of being short-lived, erratic (hot and sunny or cloudy and dimmed), and whose beauty fades? Love itself might be a useful metaphor here. When someone falls in love, the beginning is usually very passionate, but some parts of that initial passion fade over time, even if the love remains at a high level. A romantic relationship can be erratic. With intense emotions, jealousy as well as true love are possible. Love can also cause a person, who is normally subdued to do erratic or dramatic things. And, although a pessimistic view, the beauty or intensity of love (or maybe a first love as compared with subsequent love) fades over time.
Anything that is short-lived, erratic, and has qualities that fade is a good metaphor in this case. A beautiful snowfall is short-lived. It is pleasant to look at but cold to go out into. It is a beautiful sight, but as it melts, not much is left but mud and bare trees.