To start, I would have to agree with tildenm about there being a lot of body types in the world. Although there are many vegetarian/vegan ways to gain weight, which I'll list at the end for you, I think it is first important to take a look at who you are and why you may want to change something about yourself. What I'm trying to say is that trying to fit an image, either in gaining weight, losing it, or other, is pretty much impossible if you don't accept that who you are is who you are no matter what changes you do or don't go through. I'm not trying to get on you about what your motivations are; all I'm trying to say is that we're all beautiful in our form, and when we become accepting of this, it is a lot easier for change to take place. Another reason why I say this is that considering how you said that you consider yourself naturally thin, and that all of your family is as well, it is more than likely that no matter how much you eat, you may not gain a considerable amount of weight. I know this is the case with my sisters as well as many friends. I think what is important is being healthy. If your doctor believes that you are healthy, as well as yourself, then that is all the really matters. But if you still want to gain some healthy weight... I think the muscle building idea is good, just as long as you are sure to eat very well while doing so, or else you could be burning calories. Otherwise, just increase your calorie content; Foods I would suggest are avocados, breads and pastas, and nuts... and if you really just want to gain weight fast, although this isn't exactly healthy and shouldn't take up the majority of your diet, sweets of any kind will definitely make you gain weight. Hope this helps and good luck.