There is no one solution for all of us. Please tell us what diaper (nappy) you are wearing and whether you are wearing a diaper cover (plastic pants).

A sagging wet diaper often will produce chaffing, particularly in hot weather, so some means of support both helps in that regard and also covers the diaper. Wearing a pair of ordinary cotton underpants over the diaper may help somewhat. Stretchable spandex underpants work even better. Wearing an adult onesie does an even better job, but will be warmer. Another solution is to wear diaper suspenders (underwear suspenders) that support the diaper but are cooler than wearing a onesie.

Others feel that a breathable PUL diaper cover (plastic pants ) over the diaper also helps helps and is cooler than wearing vinyl plastic pants. Others find that wearing an adult onesie both supports the diaper and prevents chaffing with the thighs.

Many feel that a breathable diaper with a cloth-like outer shell will cause less chaffing than a diaper with a plastic shell. However, the diaper with the cloth-like shell will stretch and than sag considerably more than a diaper with a plastic shell.

Now you need to resist the logic to drink less water. This is a bad idea for a few reasons. First, you are going where it will be warm or even hot. You will need the fluids for sweat to help keep you cool. The other reason is your pee will get more concentrated which will cause it to have a much stronger smell and be worse for the skin. Fact is, you very well may need to me drinking more water.
What I'd do is first, if I will not be out long, let's say two or three hours then I go with a pull up. I don't normally like pull ups because I think they leak too much but I will plan on changing back as soon as I'm done doing what ever will be making me hot. For longer times out like going to an amusement park, powder is my go to item. The trick I have learned that works for me is I'll powder between the legs and get my diaper on, as normal, not too tight, not too loose and then I'll powder again between the legs. This way, I have some protection from the plastic rubbing against the skin which is what causes my chaffing. Depending on what I'm doing as to weather or not I do frequent changing (to avoid the diaper swelling between the legs and causing more pressure against the skin which causes more chaffing) or add more powder on the outside throughout the day.
Another trick might be to find some compression shorts to keep the diaper close to the body and from sagging as it becomes more wet.

Well for the heat the Diaper suspenders may be cooler, but other than that, I'm not sure how cool they really are! (Sorry for my horrible sense of humor) there's truly no one real solution. But As everyone has said. Some additional baby Powder helps me, and experimenting with different diaper related clothing always works for me. I haven't yet tried suspenders, even though JD has urged me to. Sorry JD!! I know you want me to.

Drew,

Diaper suspenders are great. I will convert Hotdogg to them some day. You may need to search for "underwear suspenders" or "undergarment suspenders." Also, just search in our archives and you will find my older postings about them.

--John