In a bowl, whisk together the soy sauce, sesame oil. agave, and water until well mixed. Stir in the scallions, garlic and sesame seeds. You can add a small drizzle of rice vinegar and or some chili flakes, if desired.
Place marinade in a large zip-top bag (or covered dish), along with the tofu and marinate for several hours or overnight, shaking and flipping the bag as necessary so that the tofu is evenly coated with marinade. It might not totally cover the tofu with marinade, but it will evenly marinate by flipping it periodically.
When ready to grill, remove the tofu from the marinade and place on a hot oiled grill. Reserve the extra marinade to serve as a sauce (or use it to marinate more tofu). You can also eat the marinated tofu uncooked, on a sandwich or diced in a salad.
Variations: To mix things up, you can add a small drizzle of seasoned rice vinegar and or some chili flakes or finely diced jalapeños. If you prefer a less sweet marinade, you can reduce the agave to 2 tablespoons.