That's a tough one. Legumes are so versatile and such a great source of protein!
You say
Legumes need longer preparation and are not always practical (and consumed daily, a bit hard on digestion system).
But, I would argue, some legumes are quite easy to prepare and eat.
Peanut butter, for example, is just right there in the jar :)
Tofu can be eaten with minimal preparation, or, occasionally, by me, with none.
Hummus - straight from the fridge onto your sandwich.
Beans and lentils are often available canned and cooked - ready to drop into a stew or just reheat and eat.
Fresh or frozen young green peas or edamame (baby soy beans) can be steam cooked in minutes.
Gram flour can be easily made into pancakes, farinata, bhajis or pakoras.
Legumes are sometimes made into pasta (though this can be expensive and is not available everywhere) and that cooks in about the same time as ordinary pasta made from semolina.
Even dry lentils, especially split and/or skinned, require relatively little preparation. For example, split skinned green gram (moong dal) can be consumed raw after an hour of soaking and a good rinse, or if not soaked, it can be thoroughly cooked in about 15 minutes (or less with a pressure cooker).
As for actual alternatives; all foods contain protein in some amount, including cereals. Seitan, for example, is made from wheat protein (gluten). These cereal proteins can also be hard on the digestive system for some people, though.
Some non-legume foods that have decent amounts of protein:
- seeds such as hemp, pumpkin and sunflower
- tree nuts such as almonds, walnuts, pecans
- grains such as quinoa, wheat, oats
- cooked mushrooms, potatoes, green veg