I'd say you have a few options.
1.) Invest in a quality heart rate monitor or some type of "Fit Bit" device to be able to calculate and record your calories burned.
2.) Locate a similar activity and swap it out in your exercise diary. So maybe choose "Walking: 3 mph" instead of mowing the lawn. Pushing the mower makes your actual activity more strenuous than just plain walking (just as pushing a stroller would..), but I wouldn't worry about it.. FatSecret tends to overestimate exercise calories anyway..
3.) Don't put the lawn mowing into your exercise diary and do a little extra happy dance when you record a bigger loss on the scale because you had bonus exercise calories that you didn't log!
Someone who is busier than you is working out right now.
There will come a day when you can no longer do this. Today is not that day.