Keats is fond of rare and archaic senses of words, but I think that all the senses here are guessable, if you think carefully about what the word needs to mean for the line to make sense in its context. Then, turning to a comprehensive dictionary, you can look up the word and check your guess.
Let’s start with the last of your examples. An eglantine is a kind of rose, also known as “sweet brier” due to its pleasant scent. So the “sweets” in line 102 must be the sweet smells of the flower. Turning to the Oxford English Dictionary, we can confirm this:
sweet, n. 6. Sweetness of smell, fragrance; plural sweet odours, scents, or perfumes. poetic.
In the next passage (lines 223 ff.), Endymion has heaped “sweets” onto the shrine and set them on fire. If we read on a few lines we find a description of what he was burning:
Bay leaves were crackling in the fragrant pile,
And gummy frankincense was sparkling bright
’Neath smothering parsley
So the “sweets” must be these sweet-scented plants: bay leaves, frankincense and parsley. Again looking in the dictionary, we find:
sweet, n. 7. plural. Substances having a sweet smell; fragrant flowers or herbs; scents, perfumes. Now rare.
Next, in “be thou cheered sweet”, Endymion has just been telling Peona to dry her tears, and so it seems that he is encouraging her to assume a “sweet” (meaning “pleasant”) disposition. In order for this to work, we need to understand “cheer” in the sense
cheer, v. 6.a. transitive (reflexive). With complement. To have or assume the specified mood or frame of mind; to feel happy, sad, etc. Obsolete.
Finally, in “ravishment its sweet”, the context is that Endymion is asking a long series of rhetorical questions. “Who, of men, can tell that flowers would bloom, that green fruit would swell, that fish would have bright mail, [etc.], if human souls did never greet and kiss?” The questions are parallel to each other, so that Keats elides some of the words in the later questions, to avoid excessive repetition. So by “or ravishment its sweet” we have to understand “who, of men, could tell [that] ravishment [would have] its sweet, if human souls did never greet and kiss?” In this context it’s clear that “sweet” needs to mean the effect of ravishment, that is “sweetness, pleasure”. Again, looking in the dictionary, we find:
sweet, n. 3.a. That which is pleasant to the mind or feelings; something that affords enjoyment or gratifies desire; (a) pleasure, (a) delight; the pleasant part of something.
