Holidays to EU countries will be possible, but they may not be as good as now.
If you’re going to the EU after 31 December 2020, then the current beneficial EU regulations on mobile roaming charges might no longer apply. Monitor your phone use to avoid potentially unwelcome new post Brexit charges.
Your EHIC card providing free essential health care cover might not be of any use anymore. Post Brexit, check your insurance cover before going to the EU.
And if you see somewhere you’d like to live or work, while on your EU holiday, then think again. UK citizens may be losing their freedom to live and work in the EU, post Brexit.
And on your way home, be careful what you buy. Post Brexit, your unlimited EU purchases might be replaced by the more restricted duty free allowances which apply to a trip to say India or Canada.