Use any nut butter, peanut, almond, cashew. Use hazelnut chocolate spread. Cream cheese (could be vegan, like I used for my toasts). Yogurt (again, vegan can work, like cashew yogurt).
Any fruits under the sun. I’m not sure if watermelon would work on a bagel toast. But hey, if you wanna put watermelon on a bagel toast and eat it, you be my guest! I like berries and bananas. This time I also tried figs, and apricots!