No, not a bike. No, not an expensive gym membership.
BUY A HEART RATE MONITOR!
Even if your exercise of choice is walking, a decent heart rate monitor can help you make the most of your time and effort. And I have to say, there is something very satisfying about seeing the number of calories burned go up!
You don't have to buy a super-expensive one, but I do suggest investing in one that has a chest strap. When I first got my Polar F11, I was worried that the strap would bother me, but it fits nicely under the band of my sports bra and I don't even know that it's there.
The HRM is also very helpful in determining APs earned. Most WW'rs follow the rule that 100 calories burned - 1 AP, and I have found this to be very accurate when I have tested against the exhertion level. Do NOT assume that the monitors on standard gym equipment are giving you the correct calories burned readings...there simply is not enough information on those machines regarding your fitness level to properly gauge this. A good HRM records your at-rest heart rate first off, and also stores other vital information to determine accurate calories burned.
I recommend Polar myself, and I know of many others who are very happy with their products. A quick Google search of "compare heart rate monitors" will give you loads of hits for great sites to review prior to purchase.
Here is a good site to help you determine where you should be working for the best results.