R.D.
Play 'school', my kids love this on rainy days!! Ask the 5 year olds teacher (if he was in kindergarten this past year) for any extra worksheets or workbooks, they just throw them out anyway. The older child can even 'teach' the younger ones. You could have art class... bust out the sidewalk chalk or paints. You could have art class... get them each a simple instrument (bongos for the baby!!)... I do this every summer with my older ones, to keep them up to speed with what they learned the past school year, and keep them entertained.
Now would probably be an awesome time to start teaching them more responsibility and chores... every summer my girls get a few new daily chores. We start this in the summer so they have a few months to practice before they go back to school and get overwhelmed ;)