What zone do you live in? You can always fill in with radishes, and bush beans grow pretty fast. Staggered planting gives you a longer harvest season.

I'm in 6a, near the coast in MA. Beans might be a good idea. They would get tall quickly.

I think next year I'll only put lettuce on the borders of my beds and mitigate the sunlight problem to some extent.

does this mean it is possible to still sow lettuce if you have enough sun? I am in 6b and have same available row space. Would love to keep sowing lettuce, arugula, mizuna, but seed packets all say planting in summer is not advised.

Generally, lettuces don't do well in summer because it is too hot. They have trouble growing or they bolt and go to seed quickly.

You can always plant some early variety carrots.