What simile does Demetrius use to characterize the shift in his feelings for Helena when confronted by Theseus and his entourage?
There's a few similes in this speech, I think. Here's the first two, in Demetrius' own words:
But, my good lord, I wot not by what power,—
But by some power it is,—my love to Hermia,
Melted as the snow, seems to me now
As the remembrance of an idle gaud
Which in my childhood I did dote upon;
And all the faith, the virtue of my heart,
The object and the pleasure of mine eye,
Is only Helena.
His love to Hermia, he says, is "melted as the snow" - so there's one simile: his love to Hermia is like snow, which melted. And also, his love to Hermia is like an "idle gaud" (a silly toy) which he might have loved ("dote") in his childhood, but now he has grown out of. So two similes: comparing his love to snow, and to a child's toy.
Here's the next bit of the same speech:
To her, my lord,
Was I betroth'd ere I saw Hermia.
But, like a sickness, did I loathe this food;
But, as in health, come to my natural taste,
Now I do wish it, love it, long for it,
And will for evermore be true to it.
This simile is of food and appetite. When he loved Hermia, it was as if he was sick, and hated a certain food (i.e. Helena). Now that he is well again ("in health"), and returned to his normal appetite, he loves that food (Helena) and longs for it again.
So three similes in total - snow, child's toy, and food. Hope it helps!