Kathy, I love the Cinque Terre. But, visiting the area without hiking would, to me, be like visiting Florence and not visiting any of the museums. It's still a lovely town, but you're probably visiting the Florence just to walk around the town.
I'm guessing there won't be as many tourists in March in any of the CT towns. I've never been there at that time of the year - not sure how much of the towns will be "open" for tourists. In season, the towns are pretty obnoxiously overcrowded with tour groups in the daytime. Maybe they won't be so bad in March. I enjoy them much more in the evenings when it's less crowded, but obviously that's hard to do when you are day tripping. I would encourage you to stick around to watch the sunset into the ocean if you can, if the weather is nice (sunset should not be too late in March, and you ought to be able to get back to Florence by a decent hour.).
The train is very easy to use to visit any/all of the five towns. Corniglia the town is a steep climb from the train station as noted above, but there is a shuttle bus that will take you up and down. The ferry boat does offer amazing scenic views on the towns, but it sounds like it doesn't start running until the end of March. Not sure what "end of March" means - check closer to your arrival to see if the ferry is an option. You don't have to reserve any of the trains (you can't and the price of the local trains in the Cinque Terre is the same no matter when you book them). You'll need to get up to one of the villages by train anyway to get on the ferry at all.