I'm not up for it.
'Non mi va' — non MEE va. The stress falls on 'mi'. Short, flat, and direct.
Use only with very close friends when you simply don't feel like doing something. This is a very honest and somewhat blunt phrase — use it where you have enough trust that no explanation is needed.
'Non mi va' literally means 'it doesn't go for me' — an idiomatic expression meaning 'I'm not in the mood' or 'I don't feel like it'. It's completely honest but can seem rude if used with acquaintances or in formal contexts.
Non ho voglia.
I don't feel like it.
Equally direct — 'voglia' means desire/inclination. Common among close friends.
Stasera non mi sento di uscire.
Tonight I don't feel like going out.
Softer version — 'non mi sento di' implies an emotional reluctance
Non sono in vena.
I'm not in the mood.
'In vena' (literally 'in vein') — idiomatic for being in the mood for something
Saying 'non mi va' to a close friend is a sign of trust — you're not bothering to make up an excuse. In Italian friendship culture, this level of honesty is valued and respected when the relationship is strong enough.