WES - Alhambra in Granada is not worth it as a day trip from Seville in my view. I visited in September a few years ago and the volume of people was horrendous. Even with beat the queue tickets you still end up waiting everywhere for the previous group to exit and even then its just chaotic. The place is undoubtedly beautiful but I would only recommend it if I was staying in Granada itself and visited literally first thing in the morning to give you some chance of seeing it properly.
I much preferred both Cordoba and the Cathedral there - still rammed with people but its so vast you still get to see large parts of it away from the crowds.
But I doubt you'll want to leave Seville to be honest - lovely city.