Gaining

Lots of Chinese food. Not real Chinese food, I mean the greasy, saucy, deep fried food they sell in most chains. I forget exactly how many calories General Tso's chicken has, but it's quite the caloric dish. I used to take the little man out for Chinese quite often. It's a shame about dairy but honestly you might have to get into pork or chicken, because there's nothing quite as fattening as pork if you refuse red meat. We don't eat pork but I know it's rather fattening.
Try to minimize movement - order your food on Grub hub or seamless or some other delivery system. Drive your car and I agree eat a lot of carbs- poundcake is a favourite snack of the little man's, pastries, etc.
One thing that also helps is eating foods which make you sleepy and taking 30 mins or so out to rest, even if you're not sleepy. It will help slow you down, and eventually you will get into the habit of eating a big meal and sleeping. Eat a second dinner before bed or in bed - a rich dessert, like brownies or cake. Or both

You may find that you can tolerate heavy cream even though you have trouble with dairy. Heavy cream has very little lactose (nearly none) and for some does not pose as issue.

Cut your calories for a few days before. I'm not saying fast completely, but if you knock a couple hundred a day off of your usual intake your body will start to think that food is short and then take full advantage of what it gets during your week alone!

Also scientifically speaking, a diet higher in protein will help you gain more fat if you are sedentary. That's just the science though, everybody's body is different. Have fun!

I have been in your shoes before where I have had a week to really make the most of my gains. What worked for me is I would get up early and make and/or eat a large breakfast ( if you don't cook, have something set aside from the night before). Later, at work have your normal routine breakfast. I found it slowed me down but the motivation that I was only doing this for a few days really kept me going. Also have a little bigger lunch. Add an additional snack or two in. If you are a soda drinker add an additional one. When you are home alone have a big dinner of your choice then later have another dinner portion size a "regular" person would eat. Before bed I found a PB and J sandwich or two helped. If you only have a few days this is doable. I promise when the week is up you will feel heavier and more distended. Numbers will show on the scale.