How about I give a choice//make some suggestions:
1. Giant scouting. NOT to fight them, but to sneak around in the mountains, try to get an idea how many there are, where exactly they're based, maybe if we're lucky find some clue about some other natural enemies they might have we could trick into fighting them for us?
2. Diplomacy-- try to talk the city council into backing a Get Rid Of These Dangerous Giants plan? (Yes, this would be an actual mission... a mission of talking and persuading)
3. Forget about the giants, and go do something useful with Gull Island now that it's ready?
4. Try to come up with a new plan to go deal with those sirens??
5. Research ancient lore for clues about 1 or 4? (Yes, this would be an actual mission-- a mission of brainy researching about monster lore)
6. Suggestions? What do you want to do, if not any of these...?