Duolingo
Good for regular practice, vocabulary building and some structures
Duolingo is not enough on its own because it does not help you put language into context, build your knowledge of the language structures in a coherent way and use what you learn productively in real conversations.
However, if you take it as an extra element of your course it can enhance your vocabulary retention and help you practice some structures.
Regular exposure and practice is key so spending 5 or 10 minutes every day with some simple exercises on the app can make a difference.
You can add your friends and family to your list of challenges for a bit of healthy competition and support. It can be fun!