Peanut butter is a perfect option. It's great for you. There are a lot of healthy mono and polyunsaturated fats in it, and the saturated fat content is very low, contrary to your post (see link). Much of the saturated fat in PB is healthy also (stearic acid and lauric acid). Additionally, they are very cheap ~$2.99-3.99 for 16 two tbsp servings. So there's your first good way to get in calories. Just buy organic and there won't be any sugar in it. Peanuts or Peanuts, Salt is what you want your ingredients to say.
You could try any kind of nut butter really. They also make sunflower seed butter, almond butter, cashew butter, etc. All great choices.
Whole wheat pasta, as you said, is also a pretty good way to get calories in. One cup spaghetti is ~175 calories, so if you had 2 cups of that, plus a piece of lean meat and some veggies, you're easily looking 500+ calories in one meal.
On the nuts comment, you really don't need to eat a lot at all, that's why they are called a calorie dense food. I.e. for every 28 almonds or so (which is probably less than a normal handful), that's about 170-200 calories.
Don't get a high calorie drink mix. If you must, make your own. There are lots of protein powder recipes out there and healthy smoothies you can make. Find some of those. Pick yourself up some good protein, flavor of your choice.
Other good sources of calories:
- Milk, any fat percentage you want
- Greek yogurt
The items you talked about are good choices, are not hard to get, really aren't expensive, and are pretty calorie dense. It seems to me like you have or have had a previous eating issue. If that's true, what you need to do is just learn to eat more, however it is you go about that. Basically that's the problem here, because quite frankly, 2000 calories is not hard to do, and it's certainly not at the point where you should be worried about how you're going to get that much in during the day, especially if you're active.