Eden Project
If you only have one rainy day in Cornwall and one plan to make, this is the plan. The Eden Project's biomes — two vast geodesic domes built in a disused china clay pit near St Austell — are warm, humid, and completely weatherproof. The Rainforest Biome is 35°C inside regardless of what's happening outside, and the Mediterranean Biome smells of lavender even in January. What makes Eden work on a wet day is not just the shelter — it's the contrast. You walk in from a grey Cornish car park and within five minutes you're standing under a canopy of banana palms. The outdoor gardens are worth walking even in drizzle (bring a jacket), and the cafe at the top of the pit serves the best coffee on the south coast. Book in advance in school holidays; the rainy-day queue is the longest queue in Cornwall.
Near St Austell · All-weather · Allow 3–4 hours · Advance booking essential in holidays · Cafe and restaurant