I really wouldn't advise doing this. You aren't going to be able to walk into most restaurants and be seated quickly. March can be quite busy. If you don't want to plan, then don't purchase the dining plan. (It doesn't really save you money, anyway.)
What worked well for our last trip, was to book our 3 FP+'s around 10am, 11am, and 12pm. Then, I booked a ADR for lunch around 1pm. After a busy morning, it was nice to sit down. Most of the restaurants have the same menu's for lunch and dinner now. After lunch, our day was open. We could stay in the same park or switch to a different one. We'd grab dinner at a quick serv when we were hungry. (We were not on the dining plan.)
I don't think you can go to WDW anymore without some planning.