I'd like to immediately terminate my internet service with COmcast and arrange for the return of the leased modem/router. What's the best way to do that?
You can call, but the most effective way is to carry Comcast's equipment to the nearest local store and turn it back in to them. Be sure to get a receipt, and keep it forever. Also consider scanning or copying it; my shop uses thermal paper, which fades to a blank piece of paper in a week or so.
You can also take your equipment with you to a UPS Store and an associate will pack and ship your equipment.
Bear in mind you may be under a contract and thus subject to an early termination fee. The official word on early termination fees: http://forums.xfinity.com/t5/Billing/Early-Termination-Fees-ETF/m-p/2895117#M96760