Congratulations on your first event! I must say I envy you in a very positive way .
Since it is a Portugal theme, you can use the colours red and green (the colours of the national flag) as main colours in the decoration. Also, some wine bottles can be incorporated, or wine corks to be used as part of the centrepieces - after all, wine is very popular and often associated with Portugal. Also, roosters - a very typical symbol of Portugal. You can use it in many different aspects of the decoration. I have recently read about a festival in AGUEDA, PORTUGAL where beautiful colourful umbrellas float above the streets.
A different approach would be to take the name Portugal (from porto - port) and use that to introduce a more sailing theme - ships, old sea maps, wooden trunks, compasses, wheels, sea food.
Depending on the activities and the customers, this could be very successful.
Good luck and let us know how the event went.