I think tablet and fizz go together by necessity. Without that chemical reaction, there's no way for a tablet to dissolve.

Endurolytes comes in a powder. I've used it on many long runs. It's unflavored and unsweetened, so the taste is a little bit bitter and metallic (plus it's pretty low in salt, so you still need to add a pinch of table salt). But I find that once I get electrolyte-depleted, I don't notice the taste. My body asks for what it needs.

Skratch Labs also comes in a powder, either in bulk or in single serving packets. I've used that too. Since even the "everyday" formulation is flavored with a little bit of sugar rather than sugar alcohols/stevia/etc., you do have to be extra careful about cleaning your equipment to avoid mold problems.