If you're a fan of taxidermy/curiosity stores like Paxton Gate in San Francisco or Deyrolle in Paris, you'll need to check out the whimsically morbid Belfry Oddities. It's located in the Pioneer Square area near the train station, but don't let the slightly rough neighborhood put you off. Inside, you'll find a fascinating assortment of taxidermy mounts, carnivorous plant terrariums, skulls and bones, seashells and gemstones, jewelry, hair art, and greeting cards.
While the taxidermy can get pretty expensive, there are affordable terrariums in the $30-40 range, and inexpensive trinkets like $5 glass beakers filled with tiny starfish or mineral specimens. If you feel like splurging, ask about their taxidermy classes, where you can try your hand at stuffing mice, squirrels, or bunnies.