A front lip, roof spoiler, wheels, etc.
There's a lot of things that you could exterior wise

 

plenty to do... all depends on how much you're willing to spend. You can go widebody with the clinched kit if you have the means or an authentic JDM body kit.

https://www.ebay.com/itm/Lexus-IS250...YAAOSwaZJZg5IA

Maybe try a roof wing, an aggressive side skirt, and a 3IS bumper conversion. Top it off with a wrap. For inspiration I'd prob take a peak at the 2IS Post 2 pictures of your ride thread

 

 

Wald is the one that comes to mind. http://vipautosalon.com/store/index....side-skirt-set
There are reps out there for less though by Duraflex I think

 

Wheels are the biggest improvement you can do to just about any car, and a slight drop don't rice it enjoy.

 

There is an f-sport type grill that you can get. I hate the look of the stock vertical stripes grill, just haven't made the move to do it myself yet.

 

 

 

i have custom aero pieces from my car that i am selling. very aggressive

rear diffuser
front splitter
front canards
rear canards

 

Vinyl wrap your chrome window trim (or all your chrome if you wish) - fairly inexpensive and looks great
Roof spoiler will add a sporty look - readily available on ebay
There are aftermarket tail lights available on ebay as well that will freshen up the read end very nicely
As others have mentioned, aftermarket wheels are probably the single biggest visual improvement you can do to any car, lowering the car on the wheels adds to the "VIP" look
F sport grill to get rid of the vertical lines on the stock grill
Window tints - nice and dark - considering your car is black this looks really good when completed
If you want front lip and side skirt options there are many for the IS

All comes down to how much you want to spend, and how practical the mods are for you, I have a winter beater so I'm able to have a lowered rwd car with 19" wheels - I see you live in Minnesota, so lowering your car isn't always the best option due to heavy snowfalls

 

The upgrade you have done is little conventional, there are some more aggressive bodykits on the market.

 

Bright lights


Upgraded tail lights

 

There's a lot of truth to this. Ill keep going though xD