I really like this map. Very cool. Actually there are a few things I would suggest as possibilities, but it depends on the direction you want to go. Do you intend on getting into greater detail?
Designations use a wide range, I have used a lot of different styles but I do so on "feel" Try a few options out and see which one you like best overall. Think in terms of how many you want and what other elements you are going to have in the map (using letters for example on a tighter map might be less attractive if you decide to name the streets and thus can get a confusion, different colors/fonts/sizes would alleviate that confusion quite a bit)
letters and numbers are the easiest to grasp but can ruin the feel of the map. Given that you have incorporated hexes, I don't think it will spoil the overall tone whcih is an excellent mix of the old (the paper with rents and stains) and the new (the hexes).
Colors as designators can be a little more difficult, especially if you want to have a lot of variety (distinguishing Inns from brothels for example).
I have also experimented with patterns, but that has not met with a lot of success to be quite honest.
Icons can be very cool if you have a few key places and the ability/desire to draw them out. I like that, bu I am not a true artist in that sense.
Other designation helpers:
You might also want to create a grid pattern, either one which will use the existing hexes (40 or so wide) OR one that will lay over top in big squares. That way, if you have several inns noted by icon or color you can say the only inn in sector 4A or the Blue building in 6C.
Name the streets. One good way to designate is to name the streets. Most of the medieval ones I have seen have been functional (Wall street for the street around the Wall, Water Street for the one along the water etc.)
Other considerations not specifically to do with designators:
Town Purpose: With the straight (relatively speaking) streets, some of which broad, I presume that this is a trade town which has been established where the top road connects to the main road. You may want to clear out an area near the confluence of the roads (the one that runs right to left and the one that comes from the top) for a "town square" or "town commons" type area, where the traders set up kiosks, local games and pagents and feasts and such are held, that sort of thing.
Also, defense considerations: Walls, no Walls? How often (if ever) are you attacked? Where do the defenders of the town live?
Farms: I presume that all of the green area is farmland yes? How do you want the farms to interact with the city? Will the farms be 'feeders' into the city structure? Many times the surrounding villages made their living off of supplying the towns with produce etc.
Structure: Any reason that the streets along the edges of the "green zone" only have structures on one side? If I were building someothing in the city, I'd build it on the side of the street where there are no neighbors. Unless there is a sudden drop-off or something. There are a number of possibilities as to why not, but you may also want to think about making those streets along the outer edges sprout some structures -- even if they are along a wall (if you decide to have one).
Sometimes the land itself will help you out: Water sources (wells v. rivers etc.) and Topography (is this on a big hill or is it all flat) are just two examples. It all sounds very confusing, but it sometimes helps to concentrate on one aspect and let the others sort themselves out. For example, if the town is on the crest of a hill, it is not going to have a stream running from one side to the other straight through the hill (unless there is a big channel through the hill, thus a need for bridges).