Congrats on your 1st 5k!

For the 5k, I usually don't do a bag drop, like you I just bring my little water bottle and shuffle. I have seen some people do a bag drop for the 5k, but it's mostly families w/ little kids and am guessing it's stuff for the kids after the race. Disney is really good about post races, even the 5ks, you'll get a bottle of water and powerade, a snack box, a banana and sometimes bagels.
I do bring a bag for the longer races and usually I'll put in there a pair of flip flops, a shirt to throw over me post race, (but w/ DL in Sept I'm usually fine) and some post race nutrition like power bar chomps and especially w/ this race b/c weather has been on the warm side, I freeze a bottle of powerade and by the time I'm done it's defrosted and cold and awesome.