For some kids, all they ever want to do is Legos. For others, they always want to do sports. Still others, just want to be musical, or study (rare but possible : P), or read, or whatever their thing is. Each child is an individual, with individual rates of maturity, so "normal" is subjective. Does your son engage well with family or friends? That would be my main question at this age. If so, I wouldn't worry too much about what "normal" is.
Meanwhile, I have some other thoughts...How much time is your son allowed on the computer? Almost every kid I know would play on a computer for as much time as possible, if allowed. Do you think he's saying no because he knows he'll get more computer, or any kind of screen, time? Do you think he feels competent, or incompetent, in the things you're thinking up for him? Do you think it's a battle of the wills between you and him for control of his own free time and activity level? Or does he simply have a good sense of what he really wants to do?
I am a big believer in allowing kids to be kids, to allow them enough free time to play as they choose. Those quiet times are often when the stuff they learned during the day gets cemented in their brains. I also only allow one hour of computer time to encourage the play/chill/active time. And that doesn't mean I need to entertain our son in the "down time". It's his chance to direct what he wants to do.
These days, many kids are WAY TOO scheduled. It's gotten to the point where our son doesn't even like to play with some of the kids whose lives are so structured and who are signed up for everything. Why? Because many of them don't know how to play, simply play, and use their minds creatively. My son and his pals can play for HOURS with their Legos, even at age 12. (I think it helps them to think "outside the box" as they grow older which is an excellent skill to have.)
And yet, physical activity is important for overall health. When our son was younger and school work was light, I would always sign him up for one sport per season. I wanted him to get used to having at least one sport throughout the year to keep him active. A couple times he wanted to quit, but I usually didn't let him, because it was just that he didn't want to shift gears. I always gave him the opportunity to say yes or no at sign-up time. But if he said yes, i expected him to finish the season.
Even if he was not a natural athlete, he usually enjoyed playing and running around once he actually got there. I figured it was good for his body and for learning how to finish a commitment. I didn't expect perfection, but I expected general participation if he said yes at the beginning of the season.
On the flip side, our son IS a natural thinker. He excels more at school than he does at sports, and it's important he is recognizing his strengths.
As the years have passed, he has grumped more and more about the sports as his homework load has increased. This year, I finally let him forego some sports because he seemed to be making the decision from a responsible point of view. (I never thought the day would come when our son would decline something because he was gauging his workload!)
This fall, he played outdoor soccer, but has chosen to not do indoor soccer, downhill skiing, or baseball in various seasons. I knew he wasn't going to do the baseball because his best friend wasn't doing it this year (for our son, baseball is social because his skills are often surpassed by his friends). Plus, some of the sports are getting more and more competitive.
We have never let our son be involved in more than 2 things at a time--scouting and ONE sport. (Church stuff is consistently on the weekends, so we don't count the youth group events as extra-curricular.) But with school getting more complex in 6th grade, he is opting for just one extracurricular activity. Although it concerns me that he is ditching most of the sports, I have concluded he is the best judge of his level of commitment. He is continuing Scouts, which he has done for years, but for now the sports are out.
Kids are perceptive. Is your son good at the sports or is he not that great at them? Some kids are gifted with athletic talent, others are gifted in other areas. Is it possible your son is recognizing that his strengths are not in sports or the arts? Or does he realize he can quit and get back to the computer or TV if he makes a big enough stink? At 8, kids love to make a stink instead of gearing up for a sport or taking lessons (especially when they're engrossed with their Legos or have another "level" to complete on their computer game!)
If you ask yourself some of these questions, or even ask your son some of them, you might have the answer you need with your son.
Good luck!