For anyone gold league or higher I recommend using the "LOTV Multitasking Trainer" (current version 1.2) on the Arcade. It allows you to practice builds whilst requiring you to keep a probe alive that is being chased by a zergling. There are also other challenges required such as a maximum mineral limit, rescuing a unit from an island, and defending against waves of attackers. The ultimate goal is to destroy a Terran base before the timer expires. Also, the difficulty can be adjusted.
For Beginners I recommend finding a "unit tester" on the Arcade and playing around with the units to learn what they all do and test how they measure up to each other. (sorry I don't have a good recommendation since the one I used to use is no longer available) This is a really useful tool when you don't know what combination of units counters another and want to run tests.
Off topic, but related to improving, I'll go ahead and mention that Beginners should google "sc2 mechanics" to learn how to improve more quickly.