What is a good alternative to signing an NDA?

Even if your idea is not patentable, you may file a patent for it. This will give you a description and a date, so you can always prove what you knew at what point. Also, this allows you to write on the NDA that you talked about patent application 12345, which also simplifies the NDA as you do not have to fill it with details on the subject of the conversation.

In general though, be careful with NDAs. Get it drafted by an Attorney. The "gneneral" ones you find online are, most of the time, not worth the paper they are printed on.

One alternative would be to file a provisional patent application. Once your PA is filed, you can disclose any details of your technology. But ensure that your PA properly discloses your invention in detail in view of the fact that the PA will serve as priority for your later-filed non-provisional application or PCT application.

The alternative is to file a patent if the idea is patentable. Many peple are skittish about signing NDA's. Some NDA's are scarier than others and cause problems. If your NDA is causing problems consider using a less scary form. If the form is not the problem then perhaps you are dealing with someone less than trustworthy in which case find someone else.