You can do crafts with them, take them to a library for story time, take them to a playground.
I have this book (an older version though) and it is the very best resources. It has tons of ideas for young toddlers and older include lots of recipes for homemade paint, bubbles, play dough, bath paint, paper mache, play food... and it's really easy to follow with tons of great ideas:
http://www.amazon.com/s/ref=nb_ss_gw?url=search-alias%3Ds...
I teach this age range at church (as well as have kids this age) and we do a little craft, have music/singing time, action songs (like if your happy and you know it clap your hands), and we also have lessons.
I am trying to be better at this as well. I am trying to teach my sons more lessons, like science (water freezes into ice/melts at room temp), geography (like what animals live in a rainforest/desert/ which is hot/rainy...), days of the week, time (today/tomorrow/yesterday), shapes, colors, letters, numbers, puzzles, doing collages with torn pieces of paper, craft glue and things like felt cut out/foam stickers/glitter/feathers, turn on some music and dance together, do a fun excersize video together, build with blocks or large leggos, paint, play dough, bubbles...
Here are some websites I get ideas from, I know your boys are still a little young, but it is never to early to do short little lessons with them. I lack motivation, so I wrote down lots of crafts/activities and cut them out and put them in a jar, so that we can pull out a couple of ideas during the day and do that.
http://zachaboard.blogspot.com/
http://chasingcheerios.blogspot.com/
http://www.lulu.com/items/volume_6/174000/174550/2/previe...
http://urbanext.illinois.edu/babysitting/activities-toddl...
http://www.montessoriforeveryone.com/Language-Materials_e...
This website narrows down developmental/cognitive/social skills for children depending on age and has some resource as well:
http://www.babycenter.com/toddler
http://www.zerotothree.org/site/PageServer?pagename=key_p...