No reservation is required to enter Sequoia or Kings Canyon. An entrance fee is collected at the entrance stations (or buy a pass online to skip the line). Camping, lodging and cave tours do need reservations.
Wilderness permits (quota season): Reservable up to 6 months ahead. Permits release daily at 7 a.m. Pacific.
Campgrounds: Reservations are required for camping in the park's campgrounds. Book on Recreation.gov.
Crystal Cave tours: Sold only online through the Sequoia Parks Conservancy, at least 36 hours ahead. The park recommends buying 2 months before your visit. No tickets are sold at the cave.