Other than Nirvana , what else is not changing?

All compounded and/ or conditioned things are impermanent or changing. This applies to the five aggregates, physical objects, matter, energy, physical space, time, most mental concepts and ideas etc.

All matter can be broken down to energy. Energy can be converted to matter. That we know from Einstein's E=mc². Matter can convert into different forms and so can energy.

Physical space and time can be warped by matter. This we know from Einstein's Theory of General Relativity.

Nibbana is not impermanent. The Buddha's Dhamma (teachings) is also not impermanent. These things are not conditioned or compounded. They do not depend on anything else.

However, all things including Nibbana, Buddha's teachings, matter, physical space, time, thoughts, concepts are not self. There is no permanent, standalone, independent thing called self, just as you cannot find a thing called music when you breakdown a musical instrument into its constituent parts. Please also see this answer.

Element of Dhamma remains unchanging

This simply means that the teachings of the Buddha are true for all times whether a Buddha is there to reveal it to the world or not. It doesn't mean that all phenomena are permanent.

This answer is in the context of the Tibetan Gelug School and is taught in modern Buddhist Monasteries of this tradition.

Uncompounded space (as opposed to the usual idea of space) is a permanent phenomenon. This is the standard example of an existing thing that is permanent. Like emptiness, uncompounded space is taught to be a non-affirming negation. So what is uncompounded space? First, we have to understand what compounded space is...

Compounded space is the vacuum between material things. Take two material objects (made of matter) that are spacelike separated. This means not only do they not occupy the same space, but that they have a vacuity between them. Imagine they are placed in the void of deep space with a meter of distance between them. In this meter of distance between them, there is no air, there are no molecules, no matter. This is what is meant by compounded space.

Uncompounded space is the complete absence of obstructive contact. Those two things we imagined above are spacelike separated and sitting in a void. They have no contact with anything. They have a complete absence of obstructive contact. They are sitting, if you like, in uncompounded space.

This is thought of as a permanent phenomenon and not dependent on causes and conditions. How can it be said to be independent of causes and conditions? Unlike compounded space, it does not rely upon matter. Whether there is something sitting in uncompounded space or not, the mere absence of obstructive contact remains. It is unproduced.

Objection! Doesn't this mean that uncompounded space contains a self? Doesn't it mean that it is inherently existent?

Answer... No, because it is a mere non-affirming negation. It is the mere lack of obstruction. No matter how hard you look "inside" of uncompounded space you will find nothing. There is no inherently existent self in uncompounded space that can be found. This is what is meant by a non-affirming negation. There is nothing left whatsoever to be reified.

Objection! But isn't uncompounded space an independent phenomena?

Answer... No, it is dependent upon name and designation. It is empty of inherent existence just like all phenomena.

I think that most things are considered "conditioned" -- especially, anything that you perceive: any perception (including the perception of a sight, for example, but also the perception of some idea).

And anything that's conditioned is impermanent (disappears when its condition no longer exists).

I'm not sure, however, that "unconditioned" and "not impermanent" means the same thing.

In particular I don't know whether "Dhamma" (i.e. the kinds of "laws" which the Buddha taught) is considered "conditioned" -- I'd guess that the perception of Dhamma is conditioned.

I think that the Dhamma itself is described as "timeless", i.e. "akaliko", rather than unconditioned.

See also verse 183 of the Dhammapada -- the commentary says that the Dhamma taught by the Buddha is the Dhamma taught by all the Buddhas.

The Buddha said to the monk: “Conditioned arising was neither made by me, nor made by others. Whether a Tathāgata arises in the world or not, this element of dharma remains unchanging.

The above teaching is about the Law of Nature (Dhamma-Niyama) pertaining to suffering. It explains whenever suffering arises; it will always arise via the process of Dependent Origination.

This Law of Nature is permanent; just as Nirvana is permanent. However, just because both the Law of Nature and Nirvana are permanent does not mean they are the same thing; just as an impermanent rock and an impermanent cloud are not the same things.

Is it possible that Nirvana and Dhamma of conditioned arising are the same?

No, as was explained above. The logical fallacy of the question is similar to the logical fallacy of Nargajuna, who wrongly taught Dependent Origination and Emptiness are the same thing.

Dependent Origination is marked by Emptiness but Emptiness is not necessarily marked by Dependent Origination because Nirvana is marked by Emptiness but Nirvana is not marked by Dependent Origination.