These are suggestions of varieties I’ve grown in the past. Not ALL are heirlooms, but you can never go wrong with AAS Winners for production and disease resistance.
https://all-americaselections.org/winners/
Watermelon: Moon & Stars or an ‘icebox’ variety. We have very poor luck growing melons here in SW Wisconsin, so I have never had any HUGE successes in this area, other than Moon & Stars. :(
Largest, sweetest strawberries: Honeoye or Ozark Beauty
Brussels sprouts: Franklin
Cauliflower: Amazing or Snowcrown
Lettuce: I have about 3,000 ‘favorites’ in that area, LOL! If you want ‘heirloom’ go with Jung’s ‘Heritage Mix’ or, ‘Black Seeded Simpson’ or ‘Paris Island Cos (Romaine)’
Seed Saver Lettuces; I’ve grown them all, was a Test Garden for them for many years. Start here:
https://www.seedsavers.org/quick_find/lettuce-looseleaf
Good Luck! :)
Try
http://www.territorialseed.com/?utm_source=bing&utm_medium=cpc&utm_campaign=Branded-GSD&utm_term=territorial%20seed&match=e&r=PPCBINGB&msclkid=d29b92c586821c2e721ec8a1b46a514e
if you garden in the far west...