Something that is getting more and more common at theme parks and zoos near us (
Disneyland, Knotts Berry Farm, LA Zoo, San Diego Zoo and Safari Park) are these lightweight devices that can attach to pretty much any manual wheelchair and make it a power chair. There are a lot of different types now, but the one that seems to be most common for the parks is the EZRIDE+, which everyone DH has spoken with says has power to spare at the end of a park open to park close day, even on an 8 AM to Midnight day at Disneyland.