Generating some sort of barcode, stored in the system (i.e. image) as well as being presented to the visitor on receipt of payment, with the nice text "If you print this yourself and bring it to FSCONS, your registration will take less than 30 seconds, instead of up to 3 minutes, wouldn't that be nice?" (or something like that) ;)
If the barcode is a 2D barcode (e.g. QR Code) they would be readable by common Java-enabled cellphones and no peripheral equipment necessary. It would need to contain some id-number other than the name of the visitor (for lookup in the visitor database) which matches the correct name etc. (anti-scamming) but overall it shouldn't be that complicated.