there are lots of things that are knit flat. On circs you CAN knit in the round, sometimes. For most the knitting in the round you use dpn (or circs, or magic loop on dpn, or 2 circs of the same needle size)... all that.
You can make most pullovers, jackets, and so on on straight needles.
So: your limitations are small.
There even is a method for knitting in the round (seamlessly) on straight needles. If you want to do that: check youtube, I think there are instructions on there.
I would recommend a hat or mittens since they are quick and fit the season - maybe make them matching your scarf? Then it is a ful winter set.
I guess you may be more limited by yarn stash than needles?