I bought a 5 pack of white onesies from target that were really affordable; all 5 of them were the price of 1 at a craft store. I went for the 3 month size and not newborn. If you look at the weight for a newborn size it ends at about 8 pounds and here's what I've learned: if a baby is born under 8 pounds they don't stay that small for long, and having a new mom struggle to get a tight onesie on a squirmy baby is a tricky thing, so go for the 1 size bigger to be nice and let the gift last a little longer.
I got a pack of fabric markers at Hobby Lobby and cut my own little cardboard cutout to make it sturdier. It was a little small so I used masking tape to hold them on a little tighter. The markers were super easy to use, but as you can see on the rocket picture the lines stay when you're coloring in. I think outlines or just writing would be best.
After 24 hours I washed the onesies and folded them all up for my gift! I have a few ideas for my other blank onesies for gifts for friends. Keep your eyes out for some other baby gifts on here!