Back to what I was saying, decent enough rewards are very easy without even touching the in-game purchase things. Just give something that people want but is not on sale... For example:
- Free lev 20 artifact with "perfect" stats among a few ones to choose from....
- Free furniture using models that are already in the game but never used for the serenitea items (coff coff the Klee themed furniture from that island....)
- Free Paimon pet
- Free ascensions item set for a single character of your choice (choose a char, get every item needed to ascend to lev 90)