Just remembered a quick and yummy snack that's quite light:
Get a bottle (about 250mls) of your fave smoothie (I love innocent strawberry and banana) and simply stir it with full fat bio yoghurt in a big glass to make about 500mls. This can provide about 425 calories but it doesn't taste all fatty and puddingy like the typical milkshake concoctions. Sometime I add a tsp or two of flaxseed oil to it which makes it about 500kcals. Very high in calcium and slow release carbs with vit C and fibre, too!
Also I have discovered that tahini (ground sesame) is even better than butter with marmite on toast, so a generous spread of that gives about 90 kcals, and is packed with protein, healthy fats, and calcium.
I have two slices of toast (200kcal) with tahini (180) and marmite. Thats 380 delicious and healthy calories and it's not that filling.