Even tinyUSBboard can be used standalone with just one USB-capable computer necessary, you might want to extend playing with it.
Some useful first-step hardware addons are collected here:
In order to use tinyUSBboards PIN-header to connect further circuits: Solderless Breadboard
...and you might need also a set of Breadboard cables for it
Some crappy cheap digital multimeter...
...and maybe also some crocodile clip wires
Perhaps some RGB LEDs...
... and a LCD module
Maybe some cheap ethernet network interface card for ATmega8's SPI interface (you may need 3.3V voltage)
For soldering may be ZD-99 station
...in combination with some solder sucker
For soldering you may also need solder wire. Therefore Felder Sn60Pb40 with lowest diameter (<= 1mm) possible is recommended.
For further suggestion please contact me.