I love pairing bitters with my sodastream! A few of my favorite recipes are: 1) Grapefruit & Cherry, 2) Cardamom, and 3) Orange & Cranberry. Clearly it's all to taste, but a 3-4 dashes of each bitter is a good starting point for a ~16oz glass of seltzer.

Fee Brothers sells a 12 piece variety pack which is a great way to experiment. After that you have quite a rabbit hole of bitters brands and flavors to explore!