I need to add some weight to my frame, any suggestions?

I know it’s a nice problem to have, but apparently I overdid the excercise thing on vacation. I noticed I dropped a belt notch but didn’t think too much about it. I was walking a lot and swimming between 800 and 1200 meters a day. I was to the doctor a few weeks ago and I’m in the low 150’s. At 6’1” there’s not much in the way of reserves if I need them. I do some light weights, lots of walking, golf etc. I like being lean but this is a little light. Any other ideas?