Easy one: increase their conversion rate for google searches of keyword "blue widget" (ie google -> landing page -> credit card charged) by 5%. YOu just made them 5% of their annual turnover. Charge them a lot

Harder one: (Anything away from sales is always going to be harder to measure) Implement a simple custoemr support online chat featuire (you know the kind) - allows their support to deal with >1 person at a time, and reduces the "waiting" queue on the phone system to zero. Value? Dunno. Feel around by talking to the VP of customer service or make a judgement call like "not having to hire another five custoemr service people"