Actually, there are alot of ways to earn in splinterlands not just by playing everyday, so ill list them all here, i might not know everything so if ever you didnt see something that i havent listed in here, please tell me xD.
1: Renting out your cards
You can list your card in peakmonster or in game in splinterlands for rent, you will earn if someone picked the card that you listed in peakmonster, it depends on the card sometimes like you need to list the card that its something useful or like it has high power, heres the example.
2: Staking SPS
You can earn sps and voucher in everyday drops.
Its actually depends on the sps that you staked, the higher the sps that you staked, the higher sps and voucher you earn per day.
3: Doing your Daily Quest
Doing your Daily quest everyday is one of the easiest way to earn in splinterlands when you just buy the spellbook, everyday youll get quest every 24hrs and you can also stack your quest if youre just a renter player like me, ill probably make a stack quest guide soon so if you guys want ,let me know!
The chest contains a Legendary card but it has a lower rate than commons,rare and epics cards, and there %2 chance of the card that youll get thatll turn into gold, RNG.
also, the higher the league youre in, the higher rate you can get good cards since it high leagues has more chest compare to low leagues like bronze, and silver league is actually the comfort zone of those renter player like me.
4: Joining tournament
I suggest you guys to join in 
5: Grinding
You can earn dec by grinding in ranked games.
6: Leaderboards
Every season there`s a bunch player who climb in leaderboard and get payout when the season ends, only the top 25 gets a payout, the higher your top is, the higher dec you get from leaderboards rewards.
7: Giveaways
This one is just one of
the easiest way to earn in splinterlands by watching twitch streamers, lurking on discord general chat, coz there are some people who were doing some random giveaways like, 
Thats all for todays blog, happy earning spl fam!
If ever you are new and you dont have any acc to use as referral, you can use mine https://splinterlands.com?ref=altumns and dm me in discord so that i can give you a guide and give you some free dec and cards to start earning. thankyou and godbless everyone!