haha yeah I know what you mean, it is as difficult, if not more, as trying to stick to a very difficult diet! And most people fail at them!
For exercise, totally use weights. That is pretty much the only exercise I do other than going for walks. I do squats and all that with 3 kilo weights on each hand, had to build up to that though, i love it.
All dried fruits are good apricots (these are about 300 calories a cup) etc, although much of them are mostly high calorie from sugars. All the red meat you can handle will do you well, pasta is good if it is wholegrain- white is just the empty calories which do more harm than good, both for weight loss and weight gain. Potatoes are excellent too-- sweet potatoes even better (way more nutritious).
If you can bare it I say go crazy on nuts, haha, there is about 1000 calories per cup of macadamia, or pecan, if you can work through a cup over the course of a day you will have bumped it right up! Even adding these to your existing meals will be great.
Trick is gain weight by building muscle, gain weight by adding high calorie good fats to high protein foods, and avoid junk food! (even though it makes people gain, this unfortunately doesn't work for people underweight- payback! lol).
Yeah add the maca to the protein powder! with chocolate it tastes fine. It's bitterness is equiv to coffee, it might give a hint of taste but not a lot at all.