I haven't grown some of these. But I had good yield from Detroit Red beets. I pulled them when they were about golf-ball size. Very tasty.
I grew Super Sugar Snap peas. I only planted a few as I had been told it was too late for them. Late or not, they were very prolific and good tasting. I was sorry I hadn't planted more after all. I will this year. They seemed to be fairly heat tolerant as they produced until the temps got above 100.
I hope someone will chime in with other suggestions.
Unless you specifically want "heirlooms", you will get more responses in the vegetables forum.
broccoli - Green Sprouting Calabrese. Purple Sprouting.
Kale - Tuscan
Collard - Green Glaze, Cabbage Collard
Beet - Chioggia, Bull's Blood, Flat of Egypt.
Snap peas - don't recollect any heirlooms in this category, but there are several snow peas.
There are lots of both open pollinated and hybrid cultivars in these categories. In beets as an example, Open pollinated standards include Detroit Dark Red as mentioned, Early Blood Turnip ( my favorite), Crosby's Egyptian, Early Wonder, Ruby Queen ...
Thank you Farmerdill and glendalekid. I thought it would be nice to try an heirloom garden this year, but I'm not completely sold on the idea. However, I will definitely try growing some of these varieties!