This is a classic chicken and egg problem. You want a device that will allow you to enter small programs into a processor and you don't have access to a computer. The solution you want may require a computer so we are at an impasse.
Long ago computers had front panels. A front panel was an array of switches and lights that allowed the setting of an address register and a data register and could write the contents of the data register to memory. There are at least three approaches to the problem. There is the parallel method, there is the serial method, and there is the ISP method.
Let's try the parallel method. For the address register you are going to need 16 Single Pole Single Throw Toggle switches, sixteen LEDs, and four 4-bit parallel load binary counters. For the data register you need 8 Single Pole Single Throw Toggle switches, eight LEDs and you might as well use the counters for the data register. We wont be incrementing the data register like we will the address register.
Your homework is to select the parts, and figure out how to connect the switches, counters and LEDs together. My homework is to read the datasheet in more detail so I can see if this will be possible, without using another computer. Even if it is not, your effort won't be wasted we still need an address and data register of some kind.