The simplest Unibus card is the G727 Grant Continuity Card, which does nothing except forward certain daisy-chained bus signals from one slot to the next. You can see a picture of one on the Wikipedia Unibus page.
But a Grant Continuity Card is arguably not really a "device". I think the simplest device would be a 1-word ROM. Still pretty useless but at least it counts as a device. It would be straightforward to extend that to a larger ROM size (something between 256 and 4096 16-bit words). The smaller ROM size would be good for a bootstrap card containing instructions to boot from an external device (tape or disk), while the larger size might be good for something like a ROM-based operating system or resident monitor.
A ROM device would need to respond to Data-In requests on the Unibus, but not much else. It would not need to be a busmaster (i.e. it would not do DMA) so you wouldn't need to worry about bus grant lines (aside from forwarding them just like a grant continuity card would do).
The way a Data-In request works is basically this:
1. processor puts an address on the address lines of the Unibus, sets the C0 and C1 lines on the bus to 0, then asserts MSYN.
2. when your device sees MSYN asserted with C0=0 and C1=0 it recognizes that this is a Data-In request. if the address on the address line is one that this device should recognize, then it retrieves the requested word and puts it on the data lines of the bus. the device then asserts SSYN.
3. when the processor sees SSYN asserted, it reads the requested data from the bus. when it has finished reading from the bus, it deasserts MSYN.
4. when your device seems MSYN deasserted, it deasserts SSYN. The Data-In bus cycle has now been completed and the bus is available for the next request.