Thanks everyone for the feedback on the layout... I went with the last option (mostly because I really want a mini quilt to keep for myself!), and here it is just before I basted it. I love it!
I had a heck of a time choosing fabric for the back. The recipient is a 6 year old old boy, who loves the color red. I wanted something colorful and fun, but not too juvenile. I was really tempted by David Walker's collections, especially robots or boys will be boys, or the rocket-related prints in Sara Jane's children at play collection. But then I took one last look through my stash, and found this old (2003) Alexander Henry print of hot air balloons. I think it's almost perfect - colorful and fun, and playful but not too childish. The red borders are from a (new) IKEA sheet I had on hand. So it turns out that everything except the gray background fabric was from my stash!
And here's a lesson I learned from this quilt... I rarely press my seams open (because I'm lazy, mostly), but I did for this quilt - I didn't know what my final layout would be, so I didn't want to end up having to join a bunch of seams going in the same direction. But then I ended up having to match a crapload of open seams, which wasn't much better. I ended up with several places like this, where tiny gaps opened up at the intersections:
Ug! I went behind and sewed them up by hand before I basted the quilt, and hopefully everything will be okay once it's quilted. But next time, I'll be sure to use a thinner needle and a shorter stitch length - I think both of those would help. And no more pressing seams open for me, if I can help it!
Next up - the quilting. What would you do? I wish I had the nerve to try something fancy, but I'm pretty sure I'll just stipple it. It's quick and easy, and I know I can do an okay job with it. I picked up some variegated thread (another first for me), and I'm going to try it on the mini quilt to see how that looks...