Love the question, because I have 2. Hope you get a lot of good ideas!!

Switching the buttons is a good start. I haven't embroidered hardly any of my clothes yet even though that is why I wanted the machine. I just can't decide what I design I want to put on which item! Too many decisions to make! I think I am the one who needs help!

I just embroidered 2 denim shirt. I removed 2 sides of the breast pocket and embroidered a cordinated design and sewed it back on. Fun.

I have the freestanding lace maple shirt set from emblibrary so that will go on one. i like to buy the shirts a bit larger and wear then like a jacket when the weather gets cool.

I just finished two shirts one white and one blue (not denim), with many designs.(I should take pics of them). One with lots of flip flops and another with fish and sea creatures. I don't embroidery directly on the shirt, I do mine like patches to sew on seperatly. I embroidery on organza with tear away/ wash away stabilizer.

You could do what they call window pine and make small little windows of flowers all over the front - and across the yoke back.

I posted my two shirts on the projects page. Flip Flop shirt and front and back of my sea creature shirt.

Are they for you or your sweetie?

I've noticed the 70's designs are coming back- maybe some applique floers and words like "Groovy", Peace signs and "Flower Power"- might not be too great an idea for the guys though, you could stitch out tools or sports related designs for them. You didn't say if they were for male or female nor what age group and that will make a difference in what to choose as well. Let us know what you decide---I'm curious now! lol
Meri