What can I use instead of Sriracha if I only want to make a dish non-spicy, but similarly flavorful?
There are a lot of things you could add that would deliver a similar pepper and garlic flavor.  One thing that comes to mind is Ajvar.  It's a Serbian relish made primarily from red bell peppers, garlic, and eggplant.  I live in the US, and the local supermarket carries it in both mild and spicy versions.  Aside from being a lot milder, the flavor profile is pretty similar to sriracha sauce.

If you don't mind doing some more work, you could roast, skin, and mash some red bell peppers yourself.  Then add some garlic, a bit of something sweet (optional), and some hot pepper (also optional).