there's really only two spots i can think of that are close enough to not be a preplanned trip. Runaway Bay Quarry at the corner of w 5th ave and McKinley ave, and this small secluded waterfall off the bottom of O'Shaughnessy Dam near the zoo.
i personally have gone to the quarries since early high school, but in the past decade, so many kids have died drowning up there that they've come down pretty hard on letting anyone get into the property. you have to park across the street and walk into the complex, or figure a way out to swim up to the cliffs from the private condos' beach across the water. and getting in over there is even more complicated. once you do get to the top of the precipice though, you're good to go on a 20 foot jump with no obstructions under the water. if you walk up the side of the quarry a bit, you'll find another couple not so secluded jumpable spots, as well as the highest cliff on the quarry. with about a 40 foot drop, you also have to get a proper running start to avoid the dry rock below as well as a little under water cliff. it seems really intimidating, like most cliff jumps, until you just do it. overall, this trip is worth the hassle.
my favorite spot to go is a fairly new one to me. i can't tell you exactly how to get there as it's fairly secluded and clean and i know the people who frequent it would like to keep it that way. but, go to the O'shaughnessy Dam, and park in the lot that's available for parking, then just walk across the bridge and down into the woods. you have to cross a couple creeks and descend a small waterfall or two, but in a very roundabout way, you will arrive at this ultra-cutty waterfall with jumpable cliffs on both sides. it's not nearly as tall as the cliffs at the quarries, but it's a lot more fun and you don't have to worry much about getting thrown out or ticketed for trespassing. and the addition of the waterfall makes it a lot more relaxing to hang out around. this trip is hands down worth it, provided your wildman waterfall huntin' skills are up to par.
just don't act a fool, or get too wasted and have fun swimming this summer, kids!