One useful trick is to pre-prep your veggies after shopping. Separate the lettuce leaves, wash and dry, and store in a loose ventilated bag. Trim and wash celery sticks and green onions, seed the green and red peppers, peel and trim whole carrots, etc, and keep in plastic vegetable box.
Then, when you want a quick garden salad, you just have to grab the veggies you want, tear the lettuce, and chop the rest, and you can have a nice salad in five minutes or less, with your only clean-up being washing the cutting board and knife.
As an aside - I find some vegetables go a bit slimy or soggy as leftover salads, after being dressed. Lettuce needs to be freshly dressed, and cut cucumber loses its texture, unless it's well marinated.
As far as other good salads for keeping go...
Tomato salad: dice good fresh tomatoes, finely chop some onions and fresh herbs, and mix with olive oil, salt and pepper and a bit fo balsamic vinegar. Keeps well for a few days.
Marinated Cucumber salad: Thinly slice cucumbers and onions, and dress with white vinegar, a bit of salt, sugar, and dill. Keeps for days. I do a variation of this with lemon juice, a bit of garlic and salt and the dressing, which is more like a pickle but is a good side dish.
7-day coleslaw: you can google the recipe. A vinegar based coleslaw with grated carrot, onion and green pepper.
Greek Salad: Cucumber, onion, tomato, olives, feta cheese, a vinagrette with lemon juice, red wine vinegar, oregano, olive oil and garlic.
General Diced vegetable salad: cut up your vegetables into bite sized pieces, and dress with a vinagrette dressing. If you're using green beans, snow peas, broccoli or things like that, blanch the vegetables briefly first.
Mixed bean salad: A can of mixed beans, well drained, finely diced onion, some herbs (fresh or dried) and a vinagrette dressing. Add other vegetables if you want.
Orange Walnut Salad: Thinly slice onions, and marinate for a while in rice vinegar. Peel and segment oranges, and cut into bite sized pieces. Slice up some celery (including leaves - Chinese celery works best), and walnuts. Drain the onions and toss the ingredients, dress with olive oil and freshly ground black pepper. Also good with the addition of fresh pomegranate seeds.
Beet Salad #1: Roast or boil beets until tender. Cool, peel and cut into bit sized pieces. Dress with a basic vinagrette, and serve at room temperature.
Beet Salad #2: Same as above, but add walnuts, thinly sliced onion and blue cheese.
Beet Salad #3: Peel and grate a raw beet. Dress with olive oil, lemon juice, salt, pepper and ground cumin. Better if made the day ahead, and also good with grated carrots.
Grated Cucumber Salad: Grate seeded cucumbers (or whole small ones) and some onion. make a dressing with yoghurt, lemon juice, ground cumin, salt and pepper. Also good with fresh mint or dill added.
Chickpea Salad: Chickpeas tossed with thinly sliced celery and onion, chopped tomatoes, fresh mint, lemon juice, olive oil and a bit of garlic.
Green Bean salad: Blanch green beans, dress with your favourite vinagrette.
Asparagus Salad: Same as above, but with asparagus.
Japanese Chinese Style Cold Noodles: Julienne some fresh vegetables (like carrot, celery, cucumber, green onion), dice some silken tofu, and shred some cold chicken. Cook thin asian noodles, drain and rinse and cool. Put the noodles in a bowl, arrange the other ingredients on top. Dress with an Asian vinagrette (soy sauce, rice vinegar, sesame oil, a bit of shredded ginger).
Tuna vegetable salad: Dice cucumber, celery, tomato, onion, and whatever else you want. Mix with a can of well drained tuna, and dress with a vinagrette dressing. You can also mix this with pasta.
As an aside - if you want a creamy, healthy, non dairy dressing for meat based salads (like a chicken salad or tuna salad), silken tofu actually makes a pretty decent base. Puree it in the blender with lemon juice, garlic, and spices or herbs of your choice. A few drops of Worcester sauce works well, but I'm not sure how Kosher that is.
Chayote Squash Leaf Salad: I don't know if you can get Chaoyte squash leaves, but they're amazing if you blanch them briefly, chill, and dress with an Asian vinagrette.
As an aside, Japanese cold noodle sauce, like the kind used for Soba noodles, makes a great general dressing for noodle or tofu salads.