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I have a TI-nspire Clickpad (no-CAS) running Nspire 3.0

"3.0" ? Ndless 3.1 works only on 3.1.0.392.

What can I do with this thing called ndless?

In theory, everything: Ndless gives access to arbitrary native code (compiled C / assembled ASM), which is immensely more powerful and faster than the two other languages for programming Nspire calculators:
* a wimpy BASIC, which doesn't even allow drawing pixels to the screen... even the 20-year-old TI-81 can do that !
* a proprietary, purely interpreted Lua, which has significant two-way incompatibilities with standard Lua: it cannot even read or write files (!), and it has a proprietary event-based toolkit for keyboard / graphical / timer user interaction. The second thing is not necessarily inherently bad, it's just that Nspire programs are not portable to, or from, anywhere else, AFAICT.

Can I run a TI-84 simulator on the Nspire because I don't have the other keypad.

There's no third-party 84+ emulator, and I don't remember about a program which can launch the 84+ emulator embedded into the non-CAS OS.

What games are available on the Nspire.

For instance, nDOOM, GBC4Nspire which allows playing many GameBoy (Color) games, NESpire which allows playing many NES games