I think that's a dangerous idea, and I think that if you're not careful, that idea can bite you in the ass.
The fact is, every relationship is special simply by virtue of the fact that every relationship is unique. It is not possible for a relationship not to be unique; every person is unique, and the interactions between any set of two people is also unique. Even if I were to date a pair of identical twins, and do exactly the same thing with both of them, and take them both to the same restaurants, and have sex the same number of times in the same position with each, those relationships would be unique. There's no way for them not to be! Even identical twins aren't the same person, and the quality of my relationships arises from who my partners are, not from what we do.
Preserving some kind of unique action as a symbol of that specialness is not necessary; the relationships are special, and no two relationships are interchangeable or replaceable. The value I get from my partners has nothing to do with those things we do together; person B can not rob person A of value by doing the same things that person A does.
The danger of relying on some kind of special activity in order to make a relationship feel special comes from the fact that any sense of specialness that arises from an activity or from a symbol must always be a specialness that is fragile and unstable. If my partner feels special only because I do thus-and-such with her and her alone, then she must always know, somewhere deep down inside, that that specialness can be taken away from her; if I do that same thing with another person, then her sense of specialness is gone.
On the other hand, specialness that comes from who she is rather than from what she does can never be taken away. It's a sense of specialness that is cast in iron; it can never be destroyed and can never be dispelled; it's rock-solid, because it does not depend on anything outside of her. Nothng I do with her or with anyone else can shake that sense of specialness, because it does not rest on anything which depends on me.
Symbols are tricky things. People often confuse the symbol with the thing the symbol represents; look at all the people who want to pass an anti-flag-desecration amendment to the Constitution, for example. These people do not realize that a flag is only a symbol; destroying a flag does not damage or in any way harm the thing that the flag represents.
It's the same thing with relying on unique activities and other symbols of a relationship's specialness in order to feel that the relationship is special. If that feeling of specialness relies on some tokenor symbol of that specialness, then that feeling of specialness is vulnerable, and easily damaged; it's not a feeling of specialness that you can ever really be secure in. On the other hand, a sense of specialness that relies on no external factor is a sense of specialness far more secure.
Now, I think this is not obvious to many people, particularly to people already struggling with security to begin with. If you have invested some specific activity with your sense of specialness, and your security relies on feeling special, then giving up that specific action seems terrifying, because you may feel that if you lose this special action, you may also lose your specialness, and with it your sense of security. It's not intuitively obvious at all that you'll actually be more secure, both in your sense of specialness and your relationship, if you do not rely on some external factor to make you feel special.
But there it is. Relationships aren't always intuitively obvious.