Is there any sort of exercise I can do whilst I have Costochondritis? Even if its resistance bands or swimming or walking around the park

Give it a Rest. It is best to rest and avoid strenuous activity until it resolves. You can take NSAIDS to help with the inflammation but the condition usually resolves on its own.
You're good. Whatever is comfortable. This might even be a time to explore something new; some good aerobic exercise might actually prove anti-inflammatory and therapeutic.
Non impact. non impact exercise that keeps you breathing fairly deep but not too rapid and not hunched over should be helpful.