When is "spring"? Is it early March? Late April? If so, parks will be pretty dead. Is it late March to early April, right around Easter? In that case, it will be a bit more crowded. Crowds don't affect the zoo and Safari Park so much because you're pretty much exploring at your own pace. There are a few things that will have lines (the tram tour, the buses, the pandas), but crowds won't impact much else. Sea World and Legoland will be more affected by crowds because they have rides and shows that require lining up and waiting for. Weekends will always be busier than weekdays, so if your trip spans weekends, do the zoo and Safari Park on weekends and do the other theme parks on weekdays.