F.R.
I have a girl who is two and so my advice may not work for you.She is high energy and because she is the only child, I spend a lot of time working with her. Children learn through playing at this age, so although the computer is a really good idea, it probably wont hold the attn of a 3 yr old boy.
IMO boys are very different than girls. I think that you need to get him outside for even longer periods of time. I know you guys are going to the park, but maybe he can play in the backyard too.
At two, my daughter likes to play in her kitchen and cook me food. And she likes to get into mischief by playing in the sink using up all the soap and lotion.
Then we usually have some instrument playing(toy instruments), dancing, climbing through her toy tunnels.
So I guess my main point is that physical activity with your son and a mix of alone activity may work.
Honestly, being bored stirs creativity and imagination, so let him be bored.