Getting caps off can be challenging, and putting them back on even more so.

For toddlers, sippy cups work great. For older kids, I like the small Rubbermaid bottles that are a little bigger than a juice box and have a flip top. Whn they're a little bigger, there are also SS & nalgene water bottles with sippy straws that work great. (For the unintiated, "sippy" straws are one-way, only work when sipped on and, if well made and attached properly, won't leak. At least not until they've been abused a bit.)