Some simple plugins you could try are a plugin to change the names of players, like a nickname plugin. Maybe a simple home plugin, that'll show you how to store more complex data. Teleport plugins are pretty easy to make, if you want to challenge yourself, make a teleport plugin that prompts a user to accept the teleport. Other than that, create a server and try to code all the plugins you need yourself. That will really challenge you and gives you incentive to keep coding.