Since the front panel board I am using is actually a reproduction board by
Mike Douglass, creator of the Altair 8800 Clone computer and namer of the
Altair 8800c - different things! - you can output to the data LEDs!

During normal operation, the data LEDs will simply display whatever data is on
the currently selected memory address. Douglas' board, along with its many other
improvements over the original MITS board, makes it so that you can toggle the
second AUX switch while a program is running to make them instead display
whatever data has been sent out to device address 255, shared with the sense
switches.

What this means for most people is that you can use those blinkenlights!
Simply output to device address 255. In Microsoft BASIC, this is done like so:

OUT 255,(some number between 0 and 255)

You can also use hexadecimal by prefixing it with "&h". So if I wanted to make
it so that the first four lights are lit up, I'd output 0xF0 to the lights,
like so:

OUT 255,&hF0

A simple program that will count from 0 to 255 on the lights is listed below:
10 FOR I = 0 TO 255
20  OUT 255,I!
30 NEXT

The Altair is blazingly fast, though, clocked at a whole 2Mhz, so it might be
more desireable to introduce a bit of delay:
10 FOR I = 0 TO 255
20  FOR J = 0 TO 50 : NEXT
30  OUT 255,I!
40 NEXT

(While it's not necessary to use "I!" instead of "I", it's used to indicate
that the number is specifically an integer).

To convert the first character of a string to an integer and output it to the
data lights, do so using the ASC() built-in.
10 OUT 255,ASC("A")

If you want to loop through a string:
10 X$ = "Altair 8800!"
20 FOR I = 1 TO LEN(X$)
30  OUT 255,ASC(MID$(X$,I,1))
40 NEXT