Having done The Eden Project in the rain, I'd say Don't. We did, It was chocka, we got drenched getting from the car park to the project and there are huge areas of it which are outdoors. Also in hindsight the humid tropical biodome was the worst place to try to dry out. Okay the day we went it really did lace it down all day, but it meant we couldn't see more than half of it.
The day after the rain we found a leaflet in a tourist information place about Rainy days in Cornwall.
I highly recommend getting splash suits too. We get one a couple of sizes too big for DD so they'll last a few years. It meant DD was fine in the rain, and the early morning dew. She was in her element splashing in all the puddles.... we just needed adult sized versions too to protect us from all her splashes!
Many thanks - I'll make sure I pack waterproofs and wellies (and pray for a little sun!) I am just worried about spending a fortune on indoor activies if the weather isn't good - but it's true, rainy beaches if you are dressed properly can be fun.
We were near Looe last year and were lucky to only have one real rainy day. As the others said, we reserved Eden Project for nice - but not spectacularly hot - weather, as it was so lovely wandering round the outside gardens.
On that one rainy day we went first to Carnglaze Caverns, which is very close to Looe. It was actually quite magical wandering round the 'enchanted dell' afterwards in the rain, but it helps to have good raincoats. After that I'm afraid we capitulated totally to capitalism and went to the Trago Mills shopping centre and bought some outrageously discounted clothes.