Can anyone recommend what to do after completing all of the topics? I would like to continue learning not just practising.
Any ideas from other members?
Finish the Duolingo tree first; that'll get you a solid knowledge of the basic grammar, and some foundational vocabulary.
After that, keep the tree gold for a bit, to let Duolingo's spaced repetition algorithms help you revise the stuff you've learned. At the same time, start reading Spanish children's books, and listening to children's radio programs. If you can get a book and audiobook of the same thing, that can really help to improve both your reading and your listening skills.
A lot of people on these forums recommend using Memrise courses to expand your vocabulary, and Lang-8 for improving your writing skills.
You'll probably also want to find someone to speak Spanish with. Have a look at this post about how to find language exchange partners: https://www.duolingo.com/comment/1444346. There are links to some useful websites on there. Don't miss DamedesLangues's comment, which has links to more sites.
It's also worth looking on the Multilingual learning resources page on the Duolingo Wiki.