I am the same as you, I have mainly done straight line quilting. But I stepped out of my comfort zone recently and did some quilted cushion covers with CURVED lines of stitching! Oooohhh…scary! Hehe. I used plates as a guide to mark curves and crescents, you can use anything round to get the size you want. On your quilt you could do ‘bubbles’ – just do lots of circles of different sizes close together, or spaced apart. I can recommend circles as it gets you out of the straight line comfort zone, but not too complicated. One tip – sew slowly!