ところで - Changing the topic of conversation by adding pertinent information with a contrasting remark. お疲れ様でした。ところで、駅のそばに新しい中華料理屋さんができたんですけど、今夜、行ってみませんか。"Thank you for your hard work. By the way, there's a new Chinese restaurant by the station. Would you like to go to night?" Going to the restaurant may be celebrating a job well done, blowing off some steam after a busy day or perhaps the speaker is treating the listener as thanks.
ちなみに - Adding something as additional information related to a principle matter. Often used in written or formal spoken language.
この人形はフランスで三百年前に作られたもので、同種のものは世界に五体しかないといいます。ちなみに、お値段は一体五万円。 "This doll was made in France three hundred years ago and it's said that there are only five in the world. And for your information, one doll is five million yen."