So you really don't need to be inside for your party, just because its cold outside.
Having a tropical themed party will defiantly bring some warmth to your cockles.
Just a few ideas on what you can have and do for your party:
* lay the floor with SAND - Yes Sand
* plant fresh or faux tropical trees around the marquee
* Thatched bespoke bars dotted in different parts of the marquee
* you could add some paddling pools, to give it that feeling
* Tropical bird noises playing in the back ground
* lay coconuts on the ground and around your trees
* have the top of your marquee roof filled with palm leaves and other foliage
* tropical punch stations
The list could go on. Below you will see the setting up of a small birthday party that i had designed and planned using sand on the ground, we also had models as sushi plates. I can make anything possible.