Any kind of circular cage works for individual trees. Small chicken wire cages could probably work for individual heads of lettuce. Deer repellent spray could probably work for sunflowers without affecting taste but not for lettuce.
This is best for individual trees:
An 8’ plastic mesh deer fence hung on 3 tensioned monofilament wires (top, middle, bottom) is not really that hard to put up. The downside is mainly aesthetic. The posts are usually just driven in the ground about 18" so they shift over time and look cheap, and the weeds are nuisance near the base.