To increase appetite you’ll want to increase your hunger hormones (ghrelin in particular), and minimize your satiety hormones (leptin, cck, peptide yy etc).
Sugar has the least satiety of any of the macros (protein has the highest satiety) so if you’re looking to simply pack in calories, sugar sweetened beverages are good (I saw an interesting study that discovered that carbonation increases ghrelin response, so most soda will give you a twofer).
Something like a donut (sugar and fat in combination has been shown to be particularly obesogenic) and you can get some of the hyperpalatability effect.
Kevin Hall recently published a study showing ultra-processed food increased ad libitum food intake by about 500 calories per day: https://www.cell.com/cell-metabolism/pdf/S1550-4131(19)30248...
Also eating/snacking throughout the day will keep insulin high and increase hunger signals and driver fat storage in your body as a result.
Of course, this basically sums up the Standard American Diet and explains why most people have the problem of putting too much weight on.
Also, as a caveat to all these “interventions,” while they are effective for weight gain, they are almost all also generally terrible for your metabolic health, and will drive MetS, diabetes, CVD, cancer, etc (basically the things killing everyone eating the SAD).