Classic game consoles are pretty useful to have. Back in the 2000s, you'd find people with Gameboys, Sega Genesis, Nintendo 64, Nintendo DS/DSI/3DS, Nintendo Wii (excluding Wii U), Xbox 360 or any Playstation up to about 3, if I recall correctly?
If you wanna display items from the 2000s, a good place to start is popular movies and books of the era. Stuff like Legally Blonde, High School Musical, Scott Pilgrim, Charlie's Angels, Scream, Hannah Montana, Wild Child, Bratz and so on. If you get a couple toys/plushies from the era, that can also help. Classic Monster High I find is a pretty safe bet, though you can also go for Tamagotchis, Littlest Pet Shop, MLP Gen 3 or Gen 4 (4 being FiM), Webkinz and so on.
If you can't do anything to the walls, they are still creative ways to express a 2000s look. You can get picture frame to put on pieces of furniture and put in whatever pictures you like, for example. There is also the possibility you will need to customise/DIY some furniture.
I also recommend checking charity shops and thrift stores with an open mind. The only limit is your mind.