The only way to get free Poptropica Credits is to complete islands (or the three games in the Store). You also get 25 credits for signing up. Otherwise, adults can purchase credits or memberships through the store.
The only two ways are completing islands (and Haunted House and Earth Day), or by buying them. That's the arrangement. There are no tricks, shortcuts, or secret methods.
You get 25 credits for signing up, and you can earn 50 by playing the free game Earth Day, or the Haunted House game, available in the Poptropica store. The only other ways to earn credits are by completing islands or by an adult purchase of credits or gift cards.
You can earn 100 free credits for completing an island, but each island only once. You get 25 credits for signing up on Poptropica, and a possible 50 for registering your e-mail with the site. You can earn 50 credits for the free games Haunted House and Earth Day (in the Store), and for Dr. Hare's Secret Lab (which costs 250 credits to play, the main draw being the pink rabbot armor).
Complete Islands you get 100 credits for each island
If you start off with no credits, then you have to buy the credits. I think $20 gets you 2000 credits. If you already have, let's say, 500 credits, than you have to buy more credits, also. You would need 1,500 credits, and 1,500c=$15.00
You can enter someone else's Multiverse room by invitation, at no cost to you. You can get multiverse with no credits if you are a Poptropica member. Paid monthly members can use anything in the store without spending credits. But they cannot keep the items used if they stop their membership.
That's the only two ways.
If you solve them you get 100 for every island. You can also finish the games in the store for more credits.
There is no way besides winning them or buying them. Unless you can get someone else to play it for you. You can also earn credits for beating Earth Day or the Haunted House, which are mini-quests available in the Poptropica store.
You can buy at least one dog costume in the store with credits. Contrary to other situations, you cannot copy purchased costumes using the Costumizer.
There is no way to get more points besides buying and finishing islands.