If I want flowers that I can plant early and that I don't have to worry about when the nights get cold, pansies are the way to go every time.
For easy flowers to plant in spring that will grow into beautiful flowers in summer I really don't think you can beat zinnias. Here is how I plant my zinnias every year to make this a chore that takes 5 minutes.
1. Select a spot for your zinnias. I like to plant zinnias in big circles so that they look like beautiful bouquets here and there in my yard. Think about a filled-in circle 3 feet across.
2. Loosen the soil in this area. I have mostly mulch beds so I have these circles exposed and have added black dirt. The mulch forms the circle around these exposed soil circles.
3. Throw zinnia seeds around. No need to plant - just toss seeds around in the circle.
4. Gently rake. I use a garden rake to agitate the soil. I don't pay much attention as to whether some seeds are buried and others are not. This works great every time. You don't need to labor over planting each seed.
5. Water whenever the soil dries out until the seedling have 1-2 sets of true leaves, at which time they should be getting pretty tough.
That's it. This planting method should take about 5 minutes once you have your zinnia spot prepared the first year. I don't worry about thinning them much but if there are too many I might pull a few. Do this and you can look forward to all-season color throughout your landscape.
By the way, profusion zinnias are awesome but I prefer them for pots. However, they do great in the ground. My favorites for the method I detailed here are "Oklahoma" or "Cut and Come Again".
The best thing about zinnias is that they are very colorful, bloom nonstop, and last for a very long time indoors when cut, so you get color inside and out. Enjoy!