Keep an eye out at Marktplatz — you're bound to come across a great travel trinket in one of its many stalls. After you've had a thorough browse of things, try some fare from one of the local food vendors.
If you're traveling on a budget, you'll be happy to discover that this community marketplace doesn't have an admission fee. Curious to know what else there is to keep you busy in and around Nidda? Head to Nidda Museum. It's close to Marktplatz and is also a tourist favorite.