Although I do and have done a lot of roomboxes with purchased pieces and/or kits, my greatest pride is in the scenes in which I make all or most of the pieces from scratch. (Not all of the pictures are available on my Fotki site due to the delay in recovering pictures in storage after Hurricane Sandy.) Some of my particular favourites are Gerry's office (my very first copy of a room), Dennis's office, the camping scene, the Leo Nickerson art room, Marie's mini display, and the penthouse.
Oh, my, I had forgotten the bar in the two-story roombox. As much as I love the detective's office on the second floor, the pieces in the bar on the ground floor are my pride and joy. The detective's office only took about a month to finish but I got totally stalled on making the actual bar for the bar room on the main floor. Got most done for it early June 2011 including the booth, cigarette machine and dart board. It wasn't until August 2012 that the bar itself finally came together in my head so I could make it and I could declare the scene finished.
One of my special tools is a small bag of Lego blocks! I have a small bag of various sizes that work for most things. They are wonderful for spacing pieces of wood at even intervals.
In the last couple tutorials, you've seen how handy Lego blocks can be as spacers in many different ways.
If I'm building shelving using butt joints Lego blocks make wonderful spacers to keep shelves level and equidistant.
If you don't have a gluing jig to keep angles straight, you can make one from Lego blocks (and build it as high or as long as you choose/need.
"Back in the day", Lego kits came with a lot of basic pieces. Now they are SO fancy that they don't generally include a lot of the basic bricks that you will want for your construction. BUT there are LOTS of other sources for the basic pieces shown above.
Because the newer kits are so specific theme oriented, your younger grandchildren may not be the best source of the pieces you will want. The best/least expensive sources will probably be thrift stores or garage sales. E-bay or Kijiji may be a great source. Search either using the words "Lego blocks". On e-bay.com I found several sellers offering up to 200 basic pieces for under $10 (didn't check shipping though).
LEGO itself has an online store that offers a shopping basket where you can order your choice of 196 diffferent Lego bricks. (Unfortunately that shopping basket link doesn't lead to the shortest and quickest choice - once you get there, click on Brick Search categories on the left hand side and that will lead you to 196 choices of the most basic pieces of Lego - and those are the ones you want.) Prices run from 10 to 30 cents per piece (and I don't know what shipping would be) so it's not cheap but you could find specific pieces (including a right angle piece I didn't know existed) .
My first choice would to be (hopefully) find a bag of pieces at a garage sale or on kijiji.