Urban board design

You have several options, I think. Do note that I probably think more in term of RPG than "fair Infinity table". You might have to adjust
1. A normal city block. A street, potentiall in the middle to create a "no man's land" kind of area, with buildings on both side as base, cover and deployment areas. Details can include cars, benches, maybe concrete blocks, lamp posts and so on. A good image to have in mind would be a police vs robbers kind of battle. Objective could be a truck in the middle of the road.

2.A city park. More open yet more closed than the city block. You can have trees, benches, Tac Rocks large and small, ponds. Perhaps you can center it around a bridge at the center over a river, creating a choke point. Or an island with an objective to reach. Perhaps an area with small stores and building is the focus. The idea is to have a mix of natural yet inhabited. Think Central Park in New York. The downside is that it might be a bit close to your forest, but the good part is that you might have leftovers to use.

3. Rich quarter. There's always an area in a city where the rich dwell. I would center this map on a single or two estate(s), as rich houses tend to be big with large properties. A good idea could be to make the map a mix or interior and exterior. The objectives would be mostly inside the estate. You can even try to make it a multi floor map. Maybe have a pool somewhere and dogs running around as hazard! No pool guy though. He ran away with the house mistress when the battle started.

4. Underworld area. The street level. This area should be thick and constraining. A place where TAGs might not be the best option. Alleyways, shady bars, clubs and warehouses. Here you can do a mix of interior and extrerior too, but this time you would have multiple small interior separated by streets and alleyway. Lot of corners to hide, large dumpsters and such. The advangtage of this map is that objectives can be anywhere.

5. Slum. Another part that's often present in city. That's where to poor and homeless dwell. This one should have run down buildings with signs of occupation, like sleeping bags, chairs and such. Buildings should be small and probably not entrable. Broken cars, trash piles and such for cover. Like the previous map, objectives could be anywhere, though due to the nature of the place, not much of value is there so you'll probably have to make up a few things that appears of value.

6. Market. I'm thinking of either a mall or an open air market here. For the first, you can refer to the estate, as in one large building with many rooms, though for this you'd likely have a large central area that is empty but with stalls and maybe a show scene, with the shops on the sides. The second would be a cluttered area full of stalls, cars, carts, produces. The trick to these areas is that they are usually not empty. This isn't such a problem but an opportunity. Decor can include actual people running around.

I'm sure I'm forgetting plenty of options. For my part, I'd lean toward 4. The Underworld area. It provides plenty of opportunity for creativity. For materials, in many cases, you'd mainly be making walls. You can use simple plastic or even cardboard to make them which should be cheap enough. You might have to purchase or get creative for decorations. Urban areas tend to have a lot of specific details that would require work to fabricate or money to buy premade.