As an avid player of D&D, finding ways to build a map and mini figures for cheap became a necessity. I've used dry-erase boards, glass chess boards, poster boards, etc.
Then I realized I had a ton of lego's in a tub. So this is how we play:
Step 1: Making the Board
Get a green platform or one around that size
Draw a grid on it (2x2 allows for a lot of "area" by making a map 80'x80'. 4x4 makes for better smaller areas like rooms and such)
Trying to get enough matching colors is a bit tough unless you already have thousands upon thousands of certain sets but for now, just use what you have.
"This bar has a rainbow colored wall..." Best line while playing with Legos
Step 2: "tiling" the Map
Setting down the flatter "tile" Legos is a good way to set differences in terrain. Blue tiles for a flowing river, black for a hole in a decrepit floor. Even building up for a stair way with a raised platform. You get the idea
If you don't have enough to do something like that then just use your imagination and use what you got.
Step 3: Mini Figures
Probably the worst part of using Legos for D&D. Mini figures are costly since they only have a few to a set and the individual ones are a tad over priced. On top of that, getting one that's unique and resembles your character is even more challenging (unless playing a Star Wars version of D&D). Finding a hood and quiver for the archer, a helm and plate mail for the fighter, you get it. You could paint up a figure. I'm looking into making custom pieces from scratch (it would be super easy if I had a 3d printer, then I could make my own entirely). But for now this is the best I can do
Step 4: Putting It All Together
The hardest part of playing D&D this way is having to build the scene on such short notice, building a warehouse takes a little time. However it's super awesome to have that set up when playing.
I hope you enjoy and leave a vote if you liked it.