We are wondering if it is worth taking a side trip to Cinque Terre or extending our stay in Tuscany/Umbria?

I don't what your other option is, but I spent 1 day/1 night there recently and if it were up do me, would have spent at least 3 WHOLE days there.

I don't know WHAT....sorry.

What time of year are you going and where are you staying in "Tuscany/Umbria" -- ?

Are you talking about a day trip or spending nights there? And how were you thinking of getting there?

Finally, what constitutes "worth it" to you?

Maybe the better way to phrase the question is: What constitutes "not worth it" to you?

I think it's definitely worth extending you trip to see Cinque Terre. I have been to many European countries, and some of my favorite memories are from Cinque Terre. It's very inexpensive and so beautiful. Riomaggiore was my favorite. I stayed in a cute apartment for 35 euros a night. There is a great restaurant that faces the water, right off the main street. can't remember the name. You can see the fisherman bring the fresh catch of the day from their boats. And the to tomatoes, you can eat them like apples!

Are you able to do mild to medium (length & climbing) hikes? Do you enjoy that sort of thing? Do you like views of the coast and the outdoors? If your answers are yes, I think CT can be very nice. I'd try to go for 2 (or 3) nights if possible so you can have most of 2 full days there.

What time of year are you going, and what days of the week will you be there? Weekends are more crowded, and summer weekends there are so many people much of the charm may be lost.