A large wooden structure at Venture in Caia Park was destroyed by the blaze, which broke out at about 03:30 GMT on Wednesday.
The main building was not affected and was operating as normal.
North Wales Police and North Wales Fire Service are jointly investigating but the cause is unknown.

Wanderers parted company with Neil Lennon after former player Dean Holdsworth's company completed a takeover of the Championship side.
Brown made more than 250 appearances for Bolton before serving as assistant manager for over six years.
"I have had no approach from Dean Holdsworth," he told BBC Essex.
"I have sat the players down this morning. We have spoken about the speculation and newspaper headlines, which inevitably you have to print.
"Speculation can take our players and my eye of the ball and that's the last thing that you need at this stage of the season."
The Shrimpers, who were promoted via the play-offs last season, are four points off the League One play-off places with 10 matches remaining.
"We are involved in winning football games and on Saturday we need 100% focus from myself, the management team, the players and the supporters," Brown added.
"If the supporters think there is a manager standing on the sidelines that will not be there in a week or two, you won't get the same level of support.
"We need it at this moment in time. It is so important that we get back-to-back promotions and get the status of a Championship side."

Ten Atlanta teachers have been jailed after investigators found wrong answers on test papers were erased and children told the right answers. The incentive - financial bonuses for good marks.
Here are five more notorious examples of exam cheating.
Red faces all round when allegations emerged in January that 34 US Air Force officers in charge of launching US nuclear missiles had cheated in proficiency tests.
According to the US Air Force, some staff had sent answers by text message to others at the Malmstrom Air Force Base in Montana.
It was a monthly test that all those dealing with nuclear missiles in the US must take.
Other officers were accused of knowing about the cheating but failing to do anything about it.
In Shaanxi province, 2,440 pharmacists were accused of cheating in a national licensing test according to China's state TV.
The scam involved the use of earpieces last October.
Fake candidates were sent into exams to get hold of the questions, the South China Morning Post reports.
They then left the test centres early to find out the answers.
Candidates who had paid the scammers the appropriate fee then received the correct answers through their earpieces.
Several hundred people have been arrested in connection with mass school exam cheating in the Indian state of Bihar.
Parents climbing school walls to pass the answers though windows may not be the most subtle method, but it's by no means the only one.
Many students smuggled in textbooks and notes into the examination centres despite tight security.
"Sports, Ethics and Religion" - the title of an undergraduate course at this US college in New Hampshire.
The ethics element seems to have been a bit lost on some students though.
Up to 64 were suspended for cheating in January, the Boston Globe reported.
At issue were electronic hand-held clickers registered to individual students to answer questions in class.
Some of these apparently fell into the wrong hands, enabling students to cheat.
Some face a one-term suspension, the college says.
Money seems to have been central to this one, according to the Cuban authorities.
Eight people, five of them teachers, were arrested last year accused of selling university entrance exams.
Thousands of of secondary school pupils in Havana were obliged to re-sit their exams, official newspaper Granma said.
An investigation has been launched following a fire in the grounds of a youth centre in Wrexham.
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Southend boss Phil Brown has said he will not be taking his "eye off the ball" despite speculation linking him with his former club Bolton.
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Dubbed one of the biggest test cheating scandals in the US, more than 170 teachers and principals were found to have helped cheating in exams in 2009.