Prism's sheet idea is a good one.
If you're like me & struggle to come up with what to quilt on those sheets -- other than a basic meander -- you might try buying a yard or two of some inexpensive cotton prints from JAF or HL. The important thing is to pick a print that you'd like to try out as a stitch pattern. You don't need to stitch every aspect of the design. If a fabric has hearts & flowers on it, but you just want to stitch the flowers, that's your choice -- or maybe just the leaves on the flowers. It's up to you! But having a print on the fabric gives you a starting point.
The other thing that I've found helpful has been marking my quilt top. Before I layer everything together, I come up with what kind of design I want. There are a couple tips I've found helpful, but they're not hard & fast rules so use them when you want & ignore them when you've got a different idea:
1) If the piecing has a lot of straight lines, adding curves creates balance (reverse is true with curved piecing).
2) Use your piecing & fabric to guide what you'll stitch. If you pieced flowers, maybe quilt some flowers or some nice leafy vines around them. If one of the fabrics in your Log Cabin design has swirls, consider quilting some swirls on your quilt.
3) To start with, pick a thread that either matches your background exactly or is slightly grayer than the main color of your quilt top. Or just pick a grey thread. A 50wt cotton thread will hide a LOT of quilting mistakes. As long as your stitches are forming, there isn't too much you could do wrong that would keep your quilt from being a great charitable donation.
4) Most importantly: your first attempts at FMQ will not turn out. They just won't. Plan to spend a minimum of 8 hours practicing FMQ before you get anything that has the remotest chance of making you happy with the results. We've all been there. There's no shortcut. You just have to practice. (I practice by making changing pads for babies. Other make quilts for pets. Neither one of those recipients will ever complain about the quality of your quilting & that practice will really help you get better!)
Once I know what I want to quilt, I grab Glad Press & Seal (or I cut up some 2 gallon Ziplock bags if I'm out of the Press & Seal) and a Sharpie marker to "audition" possible designs. DO NOT use washable markers with plastic wrap/bags as it will end up on your quilt, your hands, your clothes & anything else in sight. Once I know what I want, I grab some Crayola Ultra Clean markers & draw out my design on my quilt top. You could also use the blue quilting marker, but I have a hard time seeing those lines so I buy Ultra Clean instead. Having a line to follow really helps me focus on stitch formation rather than worrying about direction, size, style, how to connect my line and what not.
Congrats on your new machine & good luck with your first FMQ projects!