Try a simple white tee and to style it up a little, do you have a blazer? then throw on a cute necklace or bracelet.. I have some faded jeans and with it, I wear either navy scoop neck tee and my charcoal grey blazer, a slight wedge shoe and some kind of earring , necklace or bracelet..
I think it depends on how causal or not you want to be. I have also worn a tee that has a musician on it.. or even one that has a sports team.. again, throw on a cute blazer.. in my case, one of the jackets I wear is more like a suit. but it's not as stuffy looking.. OR I sometimes wear this cotton light weight jacket that has a collar and buttons down the front.. it's cute too.. and good for an evening out..
In any case, if i wear a jacket over anything, I need a more fitted shirt underneath. otherwise, the outfit looks too boxy..
Also, a cute vintage short jacket often goes with bluejeans and again, a simple tee.. they have ones at Old Navy that you can layer , I like for example how the white and navy look when you put the white underneath and it peeks out a little at the top over the navy..
with a pair of flats... it's really cute.. Also, old navy has little light weight coats to over outfits..
good luck