“Rings” were a popular suggestion for non-Moon transformation item alternatives, and I really like the idea; they’re small and inconspicuous, and you can wear them anywhere without needing a place to keep them. So, here are some design ideas for what they might look like.
Mars’s design is based on the brooch that appears on her waistband in the manga. Jupiter’s is obviously a rose, to match her earrings; Mercury’s, aside from “bubbles”, are based on her manga earrings as well. Venus’s design is taken straight from the Holy Blade, the sword from the Silver Millennium she wields in the manga.
Pluto’s design is a sideways stylized Garnet Orb with a band inspired by a key. Saturn’s is based on her amulet from the manga - the gem in the middle would be a lot taller and protrude a lot more than the others’. Neptune’s is inspired by her Deep Aqua Mirror from the manga, and her Anime lip rod; Uranus’s is also inspired by her Anime lip rod and her Space Sword.
(These guys are not to scale with each other.)