You can stay lean without any exercise simply by eating the right foods and making sure your calories in don't go over your calories out! I wouldn't recommend leaving exercise out permanently but when you have days where you know you won't be able to get a workout in then make sure you're eating the right foods and not overeating and you will stay lean! You can use this method if you're trying to loose weight but make sure your'e adding in exercise as well and that you're not consuming too few calories.
Eating the right kinds of foods is so important to staying lean and healthy. Every time you consume food you should ALWAYS try and consume a little carb, a little fat and a little protein. Fat and protein are pretty easy to manage but carbs can be a little tricky. You want to make sure you're carbs are complex and that you're not consuming too many. Remember that fruits and veggies have carbs in them already:)
One of my favorite little meal/snacks is brown rice with black beans and a dollop of sour cream(i buy the low fat because it doesn't have modified food starch) and a dollop of salsa. It's super filling and you can eat more if you need a bigger meal or less if you just need a little snack that will hold you over! And it's cheap!