Gutter cleaning-people hate cleaning their gutters, and maybe tree trimming besides the obvious mowing and raking and pruning. You could also offer mulching and tilling for gardens in the spring.

Offer to pick up all limbs/twigs in the yard.
Pick up any garbage/litter in the yard.

These will both help you in the long run as your blade will not get damaged and you won't have stuff flying all over either.

Maybe offer a service of cleaning up dog feces as well for those homeowners who have a dog. Maybe charge a little extra for this service.

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