In XMN I stayed at Le Meridien. It was a very nice property with a great lounge for breakfast, but it was a bit far from the harbour front which, as a tourist, is probably where you want to be. Next time go to XMN, I was thinking of using a Chinese property such as http://www.lujiang-hotel.com/en/index.html

I also noticed a Swiss hotel which seemed quite nice from the outside: Swiss Grand Xiamen Hotel - Swiss Grand Xiamen

I recommend you look into Kinmen Island as well. I wasn't able to make it there (can only buy tix a day in advance or something and they ran out :( ) but I've heard a day-trip there is quite interesting.

A bit of a hassle to haul luggage onto the crowded ferries to Gulangyu. Better to stay in Xiamen city and commute out there as a day trip.