I would consider something along the lines of an electric assist Xtracycle (or similar bike without the name). I wish I'd had one when my boys were little, and I'll consider getting one if I ever have grandchildren, assuming they live in the same town and I see them often.
Other than that, I'd go for a trailer, since it would be multipurpose, and they can often be found used. I bought a nice one last fall at Goodwill for $20, with the stroller conversion as well, and I don't even have a child anymore. We wanted a second cargo trailer, and as a bonus I can use the safety restraint to make it a dog trailer. The stroller conversion is nice because I can use it to walk to the grocery store and haul stuff back.
Walking is also a good option for that age. If you have or can buy a used jogging type stroller, you could use it at pick up just in case she's tired.
When we had young children we started with a double child trailer, then we got a trail-a-bike. I wouldn't want to use the trail-a-bike in busy city traffic.