It's officially summer, and with the heat comes travel and expenses like concerts, trips to the amusement park and a new wardrobe. To avoid breaking the bank, you may want to consider picking up a summer side hustle to bring in some extra cash.

CNBC Make It has talked to plenty of real people who show you can make more than just a couple extra bucks here and there — their side hustles typically bring in more then $100,000 per year.

"I've been doing [my side hustle] for about three and a half years now, and I wouldn't do anything else," Sophie Riegel, a part-time online clothing reseller, told Make It. "I love it so much. It makes me so happy."

Here are four ways you can start earning this summer and maybe even have some fun while you're at it.

1. Resell clothing

Spring cleaning doesn't have to stop when summer begins. In fact, you might be able to turn selling unwanted things into a sustainable year-long side hustle.

Riegel started selling old clothing items in her childhood bedroom as a bored Duke freshman during the pandemic in 2020. After she made around $200 selling some of her own unused stuff, she was hooked.

She began combing through thrift stores for items she could buy on the cheap and resell at a profit. In 2023, she earned $123,800 selling clothes on online marketplaces like eBay, Poshmark and Mercari.

Riegel's main job includes work as a professional writer, mental health coach and speaker. She told Make It that she spends around 25 hours a week on her side hustle.

Much of Riegel's success comes down to her research on everything from item value to restocking schedules at local stores.

"Technically, the thrifting takes the most time," Riegel told CNBC Make It. "But it doesn't feel like work to me."

2. Rent your pool

Jim Battan spent $110,000 building a pool at his home in West Linn, Oregon back in 2012. Ten years later, the pool "has paid for itself and more," Battan told Make it in 2022.

By renting his pool by the hour on an app called Swimply, Battan made $177,000 in less than two years.

Not everything goes swimmingly, Battan warned. He had to learn a lot about pool chemistry and management, and estimated checking the pool's chemicals 5 to ten times a day. It's a time-consuming side hustle, he said, but folks can do it if they put in that extra effort.

"I think of my pool as an investment and I take it really seriously," Battan said. "But there are all kinds of hosts out there that I see posting on the Facebook group that say, 'Hey, I put a bunch of chlorine in. Is that good enough?' And I think to myself, 'That's just the beginning of it.'"

3. Showcase your career through content creation

By day, Kelly Gordon is a superyacht captain. Also, by day, she spends about five hours a week creating social media content, a side hustle she told Make It brings in $10,300 a month.

Gordon stumbled into yachting after attending a wedding reception on a boat in 2007. She told Make It she "didn't even know port from starboard," but returned to the yacht the next day and asked the captain everything there was to know about the profession.

Now Gordon shares the ins and outs of yachting with viewers on Instagram. She brought in $124,000 in revenue in 2023 through sponsorships, merchandise sales, ads and affiliate marketing and public speaking engagements.

Even though she's found success creating content, Gordon isn't leaving her day job. Her side hustle works because she's still yachting.

"I'm not done running boats anytime soon," Gordon said.

4. Rent out your car

As summer travel picks up, people road tripping across the United States often look to rent cars. Andrei Echeverria's side hustle offers an alternative to traditional car rental services. He rents out his six cars through an app called Turo, which earned him up to $14,000 a month, based on financial documents reviewed by Make It in 2023.

Echeverria lives near Boston Logan International Airport, which gives him an advantage over other Turo users in finding renters. And because the app allows him to set prices, Echeverria can offer more attractive rates than the rent-a-car counters.

He spends eight to 10 hours a week cleaning, managing and booking his cars, in addition to time spent developing relationships with local repair shops. Echeverria told Make It that the success of his side hustle depends on "meeting the right people and building trust with them."

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