Trans Catalina (island) trail is good. Did it in late November for the second time. Shorter is crossing the NPS part of Santa Cruz island (shorter). Only logistics is booking the campgrounds and ferries ahead of time before they fill up.
Joshua Tree California Riding and hiking trail is an easy hike across the northern part of the park. For an end to end shuttle, try one of the outfitters near by or hike out of the Eastern end of the park (pass where you pay to get in) and Uber back to Black Rock where you don’t need to pay.
Any section of the PCT would be good except it wouldn’t be a loop, but an out and back. Just came back from a short 3 day trip near Warner Springs, great time to be hiking the warmer sections right now.
While San Jacinto and San Gorgonio wildernesses are great summer hiking, unless we are in a severe drought, they tend to still be under snow in April (San Gorgonio pk is 11.5k and San Jacinto pk is 10.8k ft).