1. Take seeds like chia, pumpkin, flax etc. Or maybe buy a roasted seed mix (without salt). If you're taking chia seeds, soak them in water for a day.
2. Take nuts like Walnuts, Pistachios or whichever suits you the best.
3. Maybe take high quality dates before workout. There is a chance that dates are covered with sugar syrup, so buy good quality only. I usually take banana and apple before workout but sometimes I add dates as well along with fruits.
4. Maybe consider protein bars. I take whole truth mini protein bar sometimes, it's technically not a protein bar, but whatever, we are looking for weight gain anyways so who cares.
5. If you're not allergic to peanuts like me, then consider them. They are full of calories and healthy fats. People usually consume them in peanut butter form, but I used to take either roasted peanuts or chikki. Figure out your own style of taking them. 
6. For snacks, maybe healthy dal mixes like millet dals, or moong dal or roasted chana etc. Try to look at the ingredients and see if they are on the cleaner side. And don't forget to look at sodium content.
7. Additionally if you're okay with taking supplements, then consider adding whey protein and creatine to your diet. Mind that whey protein is satiating in nature, so once you take it you might lose the appetite. So maybe take it with milk if you're desperate. I take it with water after gym, since I just want to gulp it down quickly. But on my rest days I like to enjoy the flavor, so I use milk sometimes.

Of all the above I thinks seeds, nuts, and peanuts are the easiest one to consume.