CT in a day (i know, i know) 3 options?

You don't share any physical limitations, so my opinion is that to go to CT and not hike is a total waste of a day (again, for someone who is able). Perusing the villages takes mere minutes. These are tiny hamlets--the views are the entire point.
Especially if it is warm out, I would hike first, then have a leisurely lunch.
You can plan your hikes with the official park website: https://www.parconazionale5terre.it/Esentieri-outdoor.php
It gives time estimates for each path.

The villages are loveliest at dusk. You'll miss that. Check the cruise ship docking schedule for the day you plan to be there. It will give you an idea of how crowded it will be.

I don't know where you'll be coming from and the dates of your trip. If you'll be there in high season, you are going to a lot of trouble for a short period of time in very overcrowded conditions.

Do keep an eye on the park's website that valadelphia helpfully provided the link for. I see that your trip is VERY soon and one heavy spring rain could temporarily close the two open SVA segments of the popular Sentiero Azzurro (Blue Path). They have just reopened after the winter season but like many CT trails are fragile + unsafe when it's wet. Hope the sun shines for you!

https://www.parconazionale5terre.it/Esentieri-outdoor.php
https://www.parconazionale5terre.it/Eiti_dettaglio.php?id_iti=3578
https://www.parconazionale5terre.it/Eiti_dettaglio.php?id_iti=3577

The hike from Vernazza to Monterossa is not that simple unless you just keep your head down and just head to your destination. However, if this is your first trip and the weather is nice, you won't be able to avoid stopping and enjoying the scenery. I would check into taking a boat from Monterrosa to Manarola if the seas are good and enjoy the views as well.

In my second trip to CT I did no hiking because some walks were closed and trained from village to village and enjoyed meandering the villages. You can see all villages in a day, a long day, and do it by train. 2 or 3 will be easy to see. The views in each village are different and all are enjoyable. Have a good trip.

Thank you...

While I wouldn't say I am fighting fit we have physical no limitations. I'm the "weakest" in the bunch and i walk 2- 3 miles a day. That said, not a fan of boats. The other two are athletic teens (sports practice 10-15 hours a week) they will be fine. They want views and beach.

Should we purchase train card in advance (while in transit) or wait til arrival. Same question for hiking card.

Any recommendations for lunch?

oh, will be there next week

Buy the CT card in advance once you're sure of your day. The line to buy in person at La Spezia Centrale station office can be vexing, or you'll have to spend time looking for a village's CT info office.
Vernazza is the epicenter of the tour groups, cruise ship and otherwise, the earlier in the day you can get there the better.
Be aware if you're going on or near one of the holidays (3/31, 4/25, 5/1) some trail sections could be one-way for several hours. Check the official website for a notice.

thank you

it doesn't look like any cruise ships will be there and we won't be there a holiday. after digging further i'm leaning toward hiking corniglia to vernazzo -- weather/closure permitting. doing other legs via train