Easy, healthy ways to gain weight?

I’m a naturally thin and relatively inactive person working in the technology field and as a student although I do move to different classes and offices for work during the day.

I used to eat quite poorly with lots of fast/junk food but now I’ve reformed- few manufactured foods, high fibre, whole grain, fruits, vegetables, no drinks but water/juice, etc. And enjoy the associated health benefits.

Throughout all of this my weight has been relatively low 120–130lbs throughout the last decade and have stick-like figure and find only W28 (or less!) pants to fit. How can I add some bulk without going Super Size Me?

I ask online because girls promptly slap me at the part where I describe how I’ve tried eating McDonald’s but just had to stop ‘cause I got sick of it and haven’t gained any weight.