I am full!

• First of all, try the simplest version: нет, спасибо, which means "no, thank you", or you can also use нет, спасибо большое — "no, thank you very much".
• If you would like to say that you are full and don't want to eat any more you can just say спасибо, я больше не хочу.
• You can make your host a compliment about quality of food and say that it is enough for you: все было вкусно, но я больше не хочу.

When you are in a bad mood

Sometimes you don't feel like going somewhere and this day you would love to stay at home, watch a movie or learn Russian numerals.

• In this case you can simply say у меня нет настроения, which means "I am not in the mood of doing something".
• Apart from that, people usually announce that they don't want to do anything right now, for example мне не хочется (+imperfective aspect of the verb, infinitive — делать домашнее задание/идти на экскурсию/готовить еду).
• One more way to express that you are unwilling to do something is у меня нет желания. Here you state that you don't have a desire to join your friends and have fun.

Uncertainty

Perhaps you have already heard that Russians have a very weird phrase to show uncertainty: да нет, наверное, and it is translated as "yes, no, maybe". Actually, people use it just to say maybe — может быть.

• If you need time to understand if you want to go or not, the following phrases can come in handy: я еще не решил(а), which means "a haven't decided yet" or я не уверен(a) — "Ι am not sure".
• If you want to say that you will think about it, use я не могу дать вам ответ сейчас because its direct translation is "I can't give you an answer now". A very common phrase is я подумаю над этим and it is for "I will think about it".

Now you know how to reject an offer in Russian in a polite way and to ask for some time to think about it. All these phrases are very typical and quite universal so you can use them in different situations when you need to refuse or reject someone's offer.