"I'm sorry, but I've already made plans (for tonight)."
That always works, because having the intention to veg out in front of the TV and then go to bed is still a plan...
However, if you want to send the message the first time that you're refusing the offer because you don't like the activity to which you are being invited, there is little you can do to take the edge off the refusal, other than to apologise and give the honest reason as to why you don't want to go.
A couple of milder ways of saying that an activity doesn't interest you include:
"Sorry, ________ isn't really my thing."
"I'm not all that keen on ________. (Though I would love to hang out with you next time you're doing something different)."
The thing about simply saying: "I'm not interested." is that it leaves it ambiguous as to what it is that you're not interested in. (I.e., are you disinterested in what your friends are going out to do? Or are you just not interested in hanging out with them? The latter is what would be considered rude.)