Looking at instruction booklets to old sets, or old LEGO catalogs and magazines.
Looking through pictures of my old MOCs, or better yet, sorting the pieces.
Anything related to F-Zero: GP Legend, LEGO Racers 2, LEGO Island 2, the Rock Raiders videogame, Sonic Advance 2, or LEGO Chess. Music especially goes.
The Old-school Veggietales videos. Those hit my nostalgia hard.
On a less faraway note, anything related to the first two years of Ninjago, the Master Builder Academy, the first year of EXO-FORCE and to some extent the Ignition Trilogy.