Easy things to keep around that make dinner prep easy & quick?

One thing I do is chop an extra onion (also celery, carrots, peppers) while I'm at it and freeze the chopped veggies in small quantities in baggies. I keep them in the freezer door. Then when I want to cook quickly, I can grab the already chopped stuff and get going.

For me, it's prepped veggies (e.g., roasted roots, washed greens) and dressings. Combine with eggs, toast, and grains for infinite meals.
Check out Tamar Adler's website and An Everlasting Meal for inspiration for cooking ahead.

For me it's about keeping a variety of quick cooking grains (quinoa, farro) and beans (canned white/black/butter beans, dried lentils, or a frozen pile of beans cooked up on sunday) frozen meats like sausage/meatballs (homemade or sometimes store bought) and always eggs. And, of course, a well stocked condiment section. Then if I can't get something fresh my garden I'll swing by the store for a vegetable. I am usually guilty of lengthy meal prep (and clean) but this week I made two quick dinners that are staples around here. First, lentils with pan fried salmon, kale and a fried egg: brown some bacon, throw in carrot/onion to brown, add a cup of lentils and 1/2 cup red wine simmering until almost dry. Add 2 cups water, a bay leaf, and simmer until done. Meanwhile, salt and pepper some salmon, add oil and cook skin side down until crisp, flip a couple minutes more. Cook kale briefly in a pan, crack eggs on top and cook until just set, flip for a couple minutes more. Top lentils with salmon and egg. Recipe #2: grate zucchini, carrot, 1/2 onion and cook in nonstick pan until starting to brown. Throw in leftover (or rotisserie) chicken. Mix 4 eggs with a teaspoon each of soy, mirin and a splash (or few granules) dashi. Mix well, pour into pan and cook until bottom is set. Broil top until done. Serve with rice or noodles, green onion/cilantro and a drizzle of oyster sauce.

I always cook ahead and freeze:
Pasta sauces, Chicken Broth, Vegetable Broth, Rice, Beans
Roast onions, fennel, garlic and keep in refrigerator for a couple of days or freeze.
Once a week prepare all you fresh vegetables and they are ready to go when you need them.
I also can and dehydrate. Right now I have canned no meat pasta sauce, pickled onions, cauliflower and beets. I have dehydrated onions, tomatoes, butternut squash and peppers.