Best food

You ask a couple very different questions. Let's tackle them one at a time...

1. Good shake to gain weight fast?

Most people on this site will cringe slightly when they read the bolded word, as almost inevitably a fast weight change is not healthy, whether gaining or losing. It sounds like you're looking to gain a more muscular look, but a fast gain will come with a large dose of fat along with any muscle. If you didn't care about what type of body mass you gained, you could simply do high-calorie milkshakes to gain weight. But if you're wanting mostly muscle, you'll want a majority of protein in your shakes, with just enough carbs and fat. Others may have specific recipes for shakes, but my favorite ingredient to include is peanut butter. Lots of flavor, ok protein, some fats from nuts.

2. Can you substitute leg press in favor of squats?

Lots of leg exercises work wonders. The squat is often termed the "king" of leg exercises, not because it can do things others can't, but because it's a one-stop-shop which covers a variety of benefits all in a single move. But if you want to avoid squats, you absolutely can do other leg exercises and still reap the same benefits. Seated leg press is a decent start, and you can play around with foot placement to emphasize different parts of the leg...feet high to work the hamstrings (back of the leg), feet low to work the quads (front of the leg), feet close or far apart to work the inside/outside of the thighs. But you do miss out on a lot of stabilizer work, and the glutes don't get the same work. Lunges can help replace much of that benefit, or you can also try a different version of the squat. Back squats (with the bar across the shoulder) are only ONE type of squat, and different versions can feel radically different in terms of preference. Front squats have the bar in front of your neck, DB squats have you hold the weights in hands hanging down beside you, goblet squats hold a weight (DB or kettlebell) in front of your face, pistol squats are bodyweight only with one leg held aloft while the other leg does all the work, hack squats are similar to "normal" squats but with a pad pressing into your shoulders instead of a thin metal bar along with a track to guide your motion and keep you from wavering one way or another, and on and on. Do some online video research for different types, maybe one will appeal to you because it eliminates the part of the squat you dislike (bar on shoulders, feeling unstable, etc).