Idea I do not like: the babysitting one.
Lara is throwing the party yes? She'd be all over that trap as protective as she is. She's too shrewd to let the one family member who finally was cured/escaped get pulled back in.
I personally consider the Summer Court gift more of a trap, since it enforces fertility. The Valkyrie gift is a close second, for at least it gives you a choice... yet it may be more devious because you could get one or the other taking sips where the other one doesn't know (like the movie 'She's having a Baby).
And for the paranoid, baby = future hostage.
But my babysitting idea has a few finer points;
1. First shows Mab (or Maeve) being a forward thinker, able to guess what the others would give and not being stuck in the past herself. I mean, magical fertility gifts in this day and age... just go to the clinic if you're having fertility problems!
2. The happy newlyweds can either use the coupon (for a free babysitting) or not. Its their choice, and shouldn't put Mab out if they never do have children. Which double-up fertility gifts would, because its kinda hard to miss that... You know, no kids running about.
3. While I guess you could use the line (Just when I thought I got out, they pull me back in!), it does show that the Winter Court is not only interested in the newlyweds bloodline (ie children), they're also interested in protecting the child's future (if only for a brief period).
4. And the funnest part is, Mab or Maeve can be totally delightful babysitters with no ill intent and still benefit. Its a total win/win for them, since they can spoil the child like a grandparent and have great fun while doing it. I mean, what Fae won't take delight in a afternoon with a mortal child.... tis so storybook like.
5. And for the future parents, they got a super safe place for the kid if things go real bad. In fact, I wouldn't include a age cut-off since Mab could see a benefit having the same child somewhat older showing-up at her door with the coupon.
But I will say, there is a pretty big downside for Mab in getting involved with whomever the pair pissed-off so much that they actually need the Winter Court to protect their kid. But then, she may actually enjoy that since she gets to belly-up to the table and get involved in a affair she normally couldn't.
6. Its a very usually gift, even a bit scary which fits the Winter Court perfectly.
7. Its a future hook, which might drive the player characters nuts as telling themselves they won't ever use the coupon (It's a sucker bet!), then end-up using it when things go to hell.