You replay all the islands and there you get credits
There are no secrets or shortcuts. You earn credits by completing islands. A parent or adult can buy credits, gift cards, or memberships through the Poptropica Store. If you want to play the Haunted House (350 credits) or Dr. Hare's Secret Lab (250 credits), and you have already completed all of the islands, you can make a new username to play islands again. However, you cannot combine credits between usernames.
For each island completed, you get the Island Medallion and 100 Poptropica Credits for the Poptropica Store. The Haunted House game awards 50 credits. There is no special prize for completing all the island quests. (see related question)
You mean on Poptropica? Well, I don't think you pay for them. But when you beat stuff on Poptropica (like islands and haunted house) you get credits. For beating islands and doing the haunted house you get 50 credits each.
As of 2013, the Haunted House is free, and you earn 50 credits. The card is an excellent tool because it lets you leave anyplace in the islands and go to the island map. (When the Haunted House was in the store, it cost 350 credits.) To beat it, see the related question.
Yes you can wait for a new Island
Once a character (username) has earned credits for all islands and the Haunted House, there are no other free credit opportunities for that username. You may create a new username, but any credits earned could not be transferred between usernames.
Until new islands come out, or games like Haunted House, there is no way at present to earn more credits. You can, however, begin a new, separate character and acquire credits for that character. Unfortunately, there is no way to transfer or combine credits among separate usernames.
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.
There is no special award, as many players have beaten all of the islands. You get the usual 100 credits awarded (50 for Haunted House and Dr. Hare's Secret Lab).
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 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.
Unless you want to buy them, you cannot get that many credits. All of the "free credit cheats" are scams or jokers. The maximum number of free credits that any single username can earn is 100 per completed island, 25 for signing up, 50 for the Haunted House, and 50 for Earth Day (in the store). The original version of Dr. Hare's Secret Lab also offered 50 credits.