RocketMiles has very limited time double miles offer for a stay by May 1, 2014, that is booked by January 18.
The offer is valid for Boston, Denver, Seattle & San Francisco and only for the first RocketMiles booking.
Note that you can combine this with the 1,000 miles sign up bonus (sign up here). There is another first time booking offer for 3,000 United miles (read more here) that would be better for some. You can access this double miles offer here.
RocketMiles has had numerous offers for the first booking. It makes sense to collect the miles that they offer for using the service once (and maybe for later bookings as well).
Here are the terms and conditions of this offer:
To be eligible, the hotel reservation MUST BE BOOKED USING THIS PROMOTIONAL LINK (www.rocketmiles.com/football-2x) by 11:59PM CT on 18 January 2014 and travel by 1 May 2014 (a yellow banner at the top of the search results page will clearly show if search criteria qualify for the bonus). Limited to first time customers and subject to investigation post-purchase. Limit one per customer. Limit one per stay. Not valid for reservations longer than 7 nights in duration. Promotional offer can not be applied to existing bookings or retroactively applied to bookings not made using the link above. This promotional offer CANNOT be combined with ANY other bonuses or offers. Rocketmiles reserves the right to retract a bonus AT ANY TIME if it detects fraud, stacking of bonuses, technical errors, cancel/rebooking activity (defined by identical search criteria), or ANY other deceptive behavior attempted to circumvent the limits expressed above.