For me, getting through a lot of milk in a day is the easiest way to get plenty of extra calories. I have no lactose intolerance so it's not hard to add an extra 1700 kcals (Cals) a day just from milk if need be. 4 pints of semi-skim is about 1100 kcals. in the UK.
I also tend to eat a couple handfuls of mixed nuts, seeds and sultanas along with a glass of milk at least once a day. Quick, simple, tasty and stuffed full of calories.