Hi! Designer and researcher, here. When Steve Blank/Eric Ries say you should "validate" your idea, this is not what they mean. This is not validating an idea. This is having a solution and looking for a problem.

To actually validate this idea, you need a few things. (Note: if all you want to do is build something for fun, then you don't need to validate anything at all! Building it for fun should be enough. Just don't expect anything more out of it, and if you decide to productize it, expect to start over from scratch.)

First, get rid of everything after "The idea is: Digital guestbook". Everything after that is way too specific. You need a hypothesis you can test, and your hypothesis is "Is there market interest for guestbooks, but digital."

Second, asking strangers here, in this way, isn't helpful. It's not providing true validation. This isn't any better than asking friends and family what they think of your idea. None of the data you get out of this (or asking your friends/family) is valid. It can't be used to support a business case.

To actually validate your idea, you need to start by making a list of places where companies already use guestbooks. The last time I saw a guestbook was a wedding. I can't remember the last time I saw one in a business. Find businesses that use a traditional, paper guestbook. Make a list of both the type of business (retail, restaurant, etc.) and the actual businesses name/location themselves.

Then, interview the business owners. Not a manager, not the cashier, but the owner, the person who is ultimately financially responsible for the entire enterprise. Ask them why they have a guestbook. Ask them what they get out of it. Ask them when they started having one and why. Ask them what they got out of it then and how that is different from what they get out of it now. Ask them about other ways they get information about their customers. Ask them to compare and contrast the utility of those methods with the guestbook. Ask them what kind of information they'd like to get from customers, but currently don't, or don't get well, or don't get easily.

Don't ask them about your digital guestbook idea at all.

Rather, interview enough business owners in different types of businesses until you start to see patterns emerging. This could be as few as a dozen interviews. This could also not be until you've interviewed a dozen people in each type of business. 2-3 is not enough, you need 8-12 people saying roughly the same thing, unprompted, on their own.

Once you have that consensus, then you can evaluate your "guestbooks, but digital" idea against it. Do "guestbooks, but digital" solve those problems?

No?

Then you were not able to validate your hypothesis. Don't pursue it.

Yes?

Then you need to see if businesses would pay you for digital guestbooks. Go back to those businesses. Present your understanding of the problem. Make sure it's correct. If it is, present how "guestbooks, but digital" solves that problem. Ask them to sign a check for $X. Do they sign a check?

No?

Then you were not able to validate your hypothesis. Don't pursue it.

Yes?

Then go build it and sell it.

That's the only true validation: will they cut you a check? That's the core part of MVP that people forget about: it's the minimum product that someone will pay for.

If they would, that's validation for your idea. If they wouldn't, you need to figure out if your solution is wrong, or if there's not a market there.

Good luck!