KEEP IT SIMPLE. (I reckon.)
you could have lists of all the work that you do, all the areas you cover, the fact that you're insured, but IMO if it says window cleaner quite prominently along with LARGE name, phone number and website, that's all that's necessary.
QR codes are good for the tech savvy, I didn't bother, but I've got one on the website and received a couple of positive comments about it. They're handy if someone wants to put your details into their phone but can't be bothered to enter it manually. They're becoming more prominent all the time..

If I did mine again I'd make the phone number and website address a bit larger, otherwise I'm happy with it, designed it myself, under a hundred squids off e-bay, stick it on yerself.. Got loads of work from it without a doubt!