'Great Lakes' Iceberg or Crisphead lettuce is a long-time favorite, providing that crispy crunch that is associated with iceberg lettuce. While you can harvest outer leaves as baby greens, it will take time for the head to reach full maturity. Fortunately, lettuces are frost tolerant, so plant these starts as soon as the soil is workable and at least 40 degrees. These can be grown in containers and honestly may be an easier way to control the leaf-munching slugs. If you choose to harvest a head all at once, cut it off about 2" above the soil and let it regrow.