How can I build up my abs without equipment or gym memberships?

I'm 16 and 6ft and I have fairly decent muscles. I've been working on my upper body and get plenty of work on my legs and thighs. What I want is to build ab muscles. I'm not looking for a 6pack and I already have the flight tight stomach. I need help with making them more tones and visible without equipment. Any ideas besides crunches or situps? Anything that'll work really would be

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