The 2,314-piece R2-D2 display model was officially retired by Lego in January, so this set was already hard to find.

One of the more impressive and enduring LEGO sets for adults (well, ages 16+, anyway) is the LEGO Ideas Tree House. It's a massive three-unit tree house you can build with either green leaves or yellow leaves for an autumnal feel. It normally rings in at $250, but it's on sale for $75 off for Prime Day. Grab it before it's gone.

Right now, the LEGO Star Wars Imperial Star Destroyer can be obtained for just $111, down from its regular price of $159. This huge 30% discount marks a new all-time low and with over 10,000 units sold in the past month alone, this set inevitably might sell out soon.