The idea of a "sprinkle" is such a cute idea!
1) I know some of your other posts are anti-favors! Personally, I like them because I enjoy seeing how other people create parties and welcome their guests. If it's a small group - a small potted plant, and a cute note, even with a paper umbrella in it and something about "thanks for coming to my sprinkle" or "something about Blooming in Spring..." Little chocolates are cute and you can melt and mold to save money - if you know how to do it. Creating a label on your computer w/ a scanned, baby picture of the Guest of Honor is very cute, even a picture, somewhere displaying the baby picture of the parent's-to-be would be cute. That label can be put on the chocolate box or other small gift. It can also be put on a bottle of sparkling grape juice, for a nice centerpiece, and given as one of the prizes or as a door prize, at the end of the event.
2) Serve a simple menu on a convenient afternoon. Set out some peanuts or small cheese tray - don't hold off lunch too long. For lunch, serve up some small tea sandwiches with fruit or salad, punch, fruit and cookie tray/small desserts or one nice cake. Other main dish ideas: Chicken waldorf salad on a bed of lettuce w/ rolls & fruit... or quiche and salad... etc.. It's elegant when you can arrange the food on each plate, especially something like the quiche or Chicken Waldorf salad, but, buffet works fine too. Outdoors would be lovely, especially at the end of May.
3) Sure... a couple games... a couple prizes. My niece had a couple fun trivia games (i.e. match the celebrity with one of their own babies... do you know Gwyneth Paltrow's or Gwen Stefani's or even Madonna's kid's names??? There are some goofy names out there!) You can even make up a trivia game about your friend - "How well do you know the Guest of honor... Questions like... "Childhood favorite toy", what she wanted to be when she grew up? (One of the parties that I hosted, the Guest of Honor wanted to be a Rock Star. If your Guest-of-Honor is into it.. the answers can be very funny!).. Good icebreaker! Just an FYI.... Someone may suggest the "candy bar/diaper game".... Some are a fan of it - I think it's a little gross!
4) Raffle tickets??? I don't know. Baby pools can be fun but the Guest of Honor doesn't really benefit from the money. She'll benefit from the gifts, though.
Have fun. It's a nice thing that you are doing.