Accept it fast, solve forward, don't compound bad luck with a bad mood. This is the entire response framework. Everything else is a variation of these three steps.

The Flight Delay

EU261 applies to flights departing from an EU airport, or arriving at one on an EU or UK carrier. For delays of two hours or more, you're entitled to meals and refreshments. For delays of three hours or more on arrival, you may be entitled to compensation of £220-£520 depending on distance. Keep all receipts for meals and accommodation you had to purchase during the delay.

More usefully: a delay is time you didn't expect to have. Download something to watch, find the least bad food option in the terminal, accept that the trip is starting slightly differently than planned, and remind yourself that you are still going.

The Hotel Room That Isn't What Was Booked

Tell the front desk immediately. Not on checkout. Now. If the room is genuinely not what was described (wrong bed type, street noise when you booked a quiet room, missing facilities), ask to be moved. Most hotels will accommodate this if there's availability. If they can't, ask for a discount on the rate and get it in writing or via email.

The mistake is not asking, enduring a bad night, and then complaining on departure. Ask on arrival when there's still time to fix it.

The Restaurant That's Closed

Every city break will have at least one planned meal that doesn't happen. The restaurant is closed for a private event. It's shut on Mondays and you forgot. There's a queue and you're too hungry to wait. This is not a disaster. Walk in a direction that looks residential and find somewhere that's clearly being used by people who live there. The spontaneous dinner is very often better than the planned one.

The Museum Shut for Renovation

Check opening status in the week before you travel. Not months before. A week before. Museums announce temporary closures for renovation with varying amounts of notice. If you arrive and find it shut, find the nearest alternative and go there instead. The second-choice museum is frequently as good or better than the one you planned. It's also usually less crowded.

Travel Insurance as the Backstop

Get it. Single-trip European travel insurance costs £10-£25. It covers medical emergencies, cancellation, significant delays, lost luggage. The one time something goes seriously wrong, it pays for itself many times over. It also removes the anxious calculation of "how much is this going to cost me" from situations where you should be focused on solving the problem.