Hello,
I am currently working on a simple smart watch using the Blend Micro and the Adafruit 0.96" OLED display. Are there any suggestions for anything else I should add?
A battery.
What are you planning on doing with your watch? I'm guessing it syncs to a smartphone? Have you considered using cellular? More complex, but would allow much more diverse to various usage scenarios. Plus, if you hope to make it a marketable product, you might open the door to an audience who would pay more for it... just a thought. 
RTC
Exactly what would I use cellular for? And what RTC would be best for my project?
How accurate do you need it to be?
Not too accurate. I want it to at least tell time reasonably accurately. What do you suggest?
My suggestion is to start the project and let us know how it works when it is finished.
If you run into difficulty, feel free to ask questions about that difficulty.
An RTC can keep time much more accurately than an Arduino, so if keeping time is part of the project I would add one, unless you can get time sync from somewhere else.
Thank you. Although I think it would be a good idea to implement something other than an RTC that is really useful? What should it be?
Well, if you have a bluetooth connection to a phone there may be a way (I have no idea how) to get the time from the phone.
It would be a lot easier with an RTC though.
I know. I plan to use an RTC. What I meant was a different function
What else should I have besides an RTC?
Well, what do you want to do with a smart watch?
At a basic level you need an MCU, an RTC and a screen.
After that you may add blue tooth or any other gadget you think you may get some use out of. Some watches measure heart rate, some atmospheric pressure...
I would like to be able to include some form of communication in order to control, send, and receive messages to and from other devices, such as Bluetooth, WiFi, cellular, etc. What would you suggest?
Firstly, how long do you want whatever battery your watch uses to last before it has to be replaced or recharged?
How much power will the LCD display consume?
Power budgets are vital for watches.
I would like the battery to last for a whole day at least. The maximum power usage for the display is 25 mA. The voltage input for the display is 3 - 5V.
OK, what exactly do you mean by a watch?
Are you talking about something that a person wears on their wrist, or something a lot bigger.
Your power requirements are extremely high for a watch.
I am talking about something worn on the wrist. The display I am using is the Adafruit OLED 0.96" breakout board.
Work out how long you want the screen to be active on a typical day, and from that, figure out how many mAh your battery will need.
You may need a strong wrist.