Rhubarb should go out as soon as possible. This is it's growing season. Choose a type that claims to be slow to bolt.

In choosing lettuces, I look for variety in color and different maturity dates so the harvest lasts longer. I don't plant iceberg at all - I've heard that it's on the low end of the scale for food value, and hard to grow well.

Plant lettuces now, and again in two or three weeks, and then again the first week of August for a fall crop.

After you dig in the hay/manure so it's mixed well with your soil, plant the first batch of seed as soon as possible.
If you plant the second batch where it gets a little mid day shade, it will do well even after the weather turns hot in June.
Lettuce doesn't like summer in Iowa unless it gets some mid day shade and plenty of water.

One bit of advice for anything you plant -- read the directions and do what it says on the package. Sounds simple, but that's the most reliable way to learn.

I planted mesclun mix this year. If you like "spring greens" it's ideal. We've been eating salads out of the garden for a month, and I didn't plant until early May! I had my first salad after 2-3 weeks, and for the past 2 weeks, I've been harvesting a salad for every dinner!