After the Western Empire's dissolution, what were the cities of Italy like? How had they changed from the cities of the later Empire (say, late 4th century)?
The Hodges and Bowden edited volume, The Sixth Century, Production, Distribution, and Demand has an excellent chapter that describes exactly what you're looking for. There's also an excellent scholarly review of the archaeology in Italian. I can get you the reference tomorrow if you want it.
Sauro Gelichi's chapter in the La Rocca edited contribution to the Oxford Short History of Italy, the Early Middle Ages is a serviceable and certainly a more accessible treatment of the subject but spans the 4th-10th century not exclusively the 6th as the chapter in Production, Distribution, and Demand does.
Many Roman cities remained in the peninsula. While certain urban centers in Piedmont were abandoned for obvious reasons, those in Emilia-Romagna remained settled. Cities in the heartland that were abandoned were often resettled later.
Where cities remained inhabited, as long as local elites were willing to keep the cities operating, they maintained many of their administrative capacities. Throughout the peninsula, however, the cities were shrinking. This process began as early as Diocletian with the reorganization of the tax system but became much more drastic as the the region was repeatedly invaded and sacked. Perhaps the most widespread and violent period occurred during the 6th century, the Gothic wars. This conflict laid waste to the countryside and cities were not the most pleasant places to live. The restoration of Roman walls during the late Antique period is physical evidence of this violence.
Within the cities, many palaces were maintained and some infrastructure was kept up as well. Aqueducts were still in operation during the early Middle Ages. For example, Brescia had a working aqueduct in the 8th century. What did disappear, more often than not, were the fora, baths, circuses, theaters, amphitheaters, and obviously pagan cult centers.
Any new buildings were often built with perishable materials and were less dense. This allowed for more open spaces and the invasion of vegetation from the countryside into the city. We shouldn't overemphasize this fact, but it did, indeed, happen.
Cities remained important despite their decrease in size. We still see panegyric written about them in the early Middle Ages and aristocracy felt the need to maintain urban residences even if they kept to the countryside mostly, this would have major ramifications for the tail end of this period as Carolingian power broke down.