My two favorite kinds of crops are cold weather crops: Asian brassica greens and peas. I love Bok Choy (white stem and green stem) and Chinese cabbage (Michihili and Napa). I've loved snow pea pods for a long time and just recently started growing snap peas (eat-the-whole-pod or "mangiatutto" peas.)
Snow & Snap Pea Pods
I've just recently started testing lettuce varieties to see what I can grow that I also like to eat. I've failed with spinach several times, I think because I didn't start it early enough in the spring.
All of those are cold weather crops, although there are lettuce varieties called "Summer Crisp," "French Crisp," or "Batavia." Some other lettuce varieties are also called "heat-tolerant." However, calling a cold-weather crop "heat tolerant" is a relative thing. Sometimes that only means that it tolerates a daily high of 75 - 80 degrees F!
If a crop does best with daily highs below 70, and your weather is going up to 90, it might be time to harvest your lettuce before it turns bitter or goes to seed. Afternoon shade and plenty of water might help a cold weather crop survive into mildly warm weather, but mostly, "plants need what they need" and our wishes come second to their nature.
Use that garden space during the summer for warmth-loving crops like beans, tomatoes, peppers, and eggplant. You could set out tomato seedlings before a lettuce patch is fully harvested.
But plan for fall crops!
Many cool-weather crops are like Chinese cabbage and do best in late summer and fall. Somehow they benefit from shortened day length and falling temperatures. Also, if your spring is short or unsettled, you might have mild and cool or cold temperatures for a longer time in the fall than in the spring.
By planning with a view to your climate, you might be able to have two crops of cold-weather plants plus a summer crop. You might need to start one crop before the previous crop is fully harvested, but that can be done if you don't need to cultivate the whole row while it's empty, to get rid of weeds. Transplanting seedlings into a bed can hasten a harvest by 2-4 weeks.
Peas, bok choy, lettuce, and spinach don't mind some periods of frost while they are deciding to germinate. They wait for the conditions they like. The main concern with sowing too early is that seed may rot in soil that's too cold or too wet. Or slugs or birds might eat the seeds. But if your summer weather is likely to hit before your spring crop is mature, giving them an early start increases the duration of your harvest, or your chances for any spring harvest.
You can direct-sow them starting a few weeks before your last expected frost date and they will grow when they can and survive most cold spells. Of course, you can throw a floating row cover or plastic film over them at night if you want to nurse them through a late, really hard freeze, or just re-sow.
Make successive sowings every 2-3 weeks until you're within 2-4 weeks of expected hot weather. The last sowings won't mature unless summer is late, but you can harvest them as baby leaves
Usually seedlings are started indoors to get warmth-loving crops into the soil earlier. But even cold-weather crops can be hastened by starting indoors. Transplant seedlings out while it's still cold, after hardening them gradually but thoroughly. It might be worthwhile trying just a few seedlings this way to learn what works well in your climate and micro-climate. You might find out that you can start something many weeks before you thought you could!
If you try some cold-weather crops, your first garden harvest could occur weeks earlier next year!