Well..There's soo many options! We got DS Disney Infinity for his Birthday in Sept and now Santa is going to get him all the extras for Christmas...
The set you go comes with 1 figure each and the "crystal" for the Incredible, Pirates and Monster Inc. world. There are additional figures you can buy for each of these worlds. For example we got DS the "villain" 3 pack for his birthday (include Syndrome, Davy Jones and Randall). They also sell other characters such as Mike W and the incredible family individually or there is also another 3 pack.
The other "world" sets you can buy - meaning you get 2 characters and a crystal are: Cars (includes Holly & Lightning), Toy Story (Includes Buzz and Jessie), Lone Ranger (includes Ranger and Tonto), Frozen (includes 2 princesses).. They also have other characters for these worlds sold individually such as Woody and Mater.
Now, there are other characters you can buy individual that can be used in the Toy Box mode (they don't have their own world crystals) they are Jack Skellington, Wreck it Ralph and Vanelope - Wreck is Ralph just came out exclusives yesterday at
Best Buy and Vanelope is due out tomorrow exclusively at Target.
So, you can buy any combination of worlds and characters you want based on which ones you think your kids will enjoy.
In addition to characters there are power discs which give your characters special powers or tools to use during the game. The discs come in 2-packs and you don't know which ones you get. Series one discs have been out and Series 2 will be out soon. The "rare" discs (like Dumbos ride car) have like a hologram look to them and if you scratch the outside of the package and one of the disc feels like it have a texture (ridges) to it then it is probably the special disc.
They also have accessories such as cases to store the figures and discs if you want.
NOW to throw another confusing wrench into it...Toys R Us has exclusivee items for Disney Infinity only at Toys R Us. They have Crystal versions and sets of some the figures...they are the same figures just with a see-thru crystal look and are supposed have a bit more power than the regular version of the characters though one of the clerks at y toys R us says not really. They also bundle some of these crystal figures in 2-packs with special power discs. The have a Crystal 2 -pack with McQueen and Buzz and a couple of discs.
Also Toys R Us has power discs that are exclusive to them. They are in a gold color package as oppose to the white/silver color of the regular infinity disc packs. Be careful in that the Toys R Us disc packs have 1 Toys R Us exclusive disc and one of the discs from the regular set and there are only so many exclusive Toys R Us discs out there. The clerks at my store tell me which Toys R Us disc ar in which packs based on bar code # so I don't waste money on duplicate discs. Like one of the Toys R Us discs is Hooks pirate ship and DS loves Hook so I got it.
So you see there's ALOT you can get! I would start with the World sets (Cars, Toy Story, Lone Ranger, Frozen) that you think your kids would like. You can always get the extra characters for the worlds they play more often (all characters can be used in Toy Story Mode). DS loves Wreck it Ralph so I got him Ralph yesterday and will get Venelope when she comes outs.
The starter kit is enough to have them playing on Christmas Day. You just add some packs for power discs and you're fine. How many extras you want to get is up to you. Just look around as different stores have different sales and such with Infinity. I've been usually getting them at Toy R Us and Target. Plus remember the stores prices match and price adjust so keep all your rceipts as items I have bought have gone on sale and I've price adjusted.
Good luck and have fun! It's a great gift and I'm sure your kids will love it.
We finally decided to just get him the skylanders instead and will send the DI back. They money we get refunded we will use for more characters. 
