Our son is a few months shy of 3 and the girls are 5 and 8. I am looking for tried and true have a pretty good attention span but if I don't set him up with something in home school room he is a maniac while the girls are trying to learn.
I find it depends on the toddler's mood. With mine, sometimes it works to read him a few board books first, and then he will sit and read over them on his own for a while, while I read to the older kids. We have our living room set up so that our couches kind of divide it, and if I get him set up with his trains or some mega blocks or whatever in the play area section, then I can often sit in the other section with the older kids. He can't really see us so he doesn't come over so often to chat/try to get other kids to play with him, and the couches keep his mess and noise contained.
We have a playpen which spends most of its time in storage, but if I pull it out now and then for him to use as a fort (I set something up so he can climb in & out and put a blanket over part of it), that keeps him busy for a long time.
Playdough holds his attention for a long time, I just have to make sure that I don't get distracted myself and then suddenly realize that he has ground a bunch into the carpet or taken out all of our cups and bowls and put a little playdough ball in each one.
Starfall.com and poissonrouge.com are helpful when he's really difficult to distract.
http://magicalchildhood.wordpress.com/ has a lot of ideas for keeping little ones busy; it used to be its own site, so I'm not sure how easy it will be to find everything, but she used to have a section with a list of ideas.
Workboxes have really worked for us. We have several that I can just get out whenever needed. They contain coloring pages, stringing beads, lacing cards, small games and activities that the kids can do on their own.
Also, people might think I'm crazy for this, but I find setting the kids up with a messy activity when I have something important to do works awesome. I usually get out the paints and let them go wild.
I am going crazy trying to homeschool my 8 and 7 year olds with a 2 yo in the mix.
2yo is into everything...
or has to potty...
or falls down...
or breaks, tears book pages, or looses pieces to various toys/puzzles while I am distracted by working with the girls.
He isn't awful- just a 2 yo.
Sorry. No solutions offered up right now.