I have one skein of yellow glow-in-the-dark yarn. What should I make?

Make a sock cap with a glow in the dark crown around it.

Or a long scarf with a bolt of lightning all through it (natural, branch-looking, or the stylized zigzag pattern)

Or knit balls to turn into Christmas tree ornaments.

You can stripe it or do color blocks (intarsia) with black. That would be really cool, I'd think.

I also think it would be fun to get some more yarn in the same color, but not glow-in-the-dark. Knit a design into the hat or scarf, and it will only show up at night.

Handwarmers would be fun in glow yarn. They don't take much yarn.

my simple handwarmers

a.k.a. fingerless mittens

using two strands of regular worsted weight yarn (weight group 4) and 6mm needles (US size 10) cast on 18 stitches (for the glow yarn, you could use one strand of that and a strand of black; that would mix well)

rib (k1,p1) for about 2" or so, longer if you like a longer cuff

switch to st st and knit about 1" to 2" more, then increase evenly one extra stitch at a time on both sides (do the increase one or two stitches in from the beginning and end of a row) for thumb

when you've increased 3 stitches on each side of each 'mitten', for a total of 24 stitches on the needle, check the height against your hand

if it's tall enough to cover your wrist and go halfway up your thumb, it's tall enough (be sure to measure it from where you want it to sit on your wrist)

cast off 4 stitches on each side of each mitten, at the beginning of each row, leaving you with 16 on each mitten

knit about 2-3 rows regular knitting (st st)

rib about 1-1/2" (k1,p1)

cast off in rib, not too tightly

sew up side seams, sew from the top down to the thumb space and knot off, then sew from the bottom up to the edge of the thumb space, leaving the cast-off 4-stitch opening open for the thumb and the top open for all your fingers