I add dried fruit and peanut butter to my oatmeal in the morning, avocado and cheese to my sandwiches at lunch and snack on things like granola + full fat Greek yogurt, trail mix, peanut butter spread on muffins. I eat a LOT of nut butters! I love Clif bars spread with peanut butter and microwaved for 20 seconds- packs in around 400-450 calories but is easy to squeeze in as a snack. I use a lot of guacamole on sandwiches and salads, and love dried apricots dipped in almond butter while watching TV :)
My tip, start working out. But do it right. If you are a skinny person you should also consult a personal trainer for the first month until you know how to do it right. After that you can do it on your own. You will be well toned, feel great and gain also a little weight.