Wear good walking shoes. Walking is the best way to explore Lahti. Forget the vanity :-) I speak from my own experience! Umbarella might be useful too in summertime.
Toiletries and Medical Supplies: I would needed plasters because I was so vain :-)
If you come here in wintertime, be sure to have enouht warm clothes. It can change quicly from -5¤C to 25¤C, well not very often, but you never know when.
In summertime I recommend to make trips to nature and then it might be good to have rubberboots with you, or some other shoes, that you don't care if they go wet or dirty. It can be sunny in the morgning and rainy in the evening.
Camping/Beach/Outdoor Gear: Be prepared for changes of the weather. You might need bikinis and raincoat during the same day.
You dont need anything special when visiting Lahti. In winter obvioualy you will need to dress sensibly!!
Photo Equipment: Definitly bring a camera. Its not the most exciting city but theres plently of areas worth photographing.
Winter gear of course.
Winter shoes, warm underwear, woollen hat, gloves, scarf etc.
Photo Equipment: My pictures here are not that well as I had no zoom, so if you have one, pack it too