Ivy Cube, Dino Cube (both Qiyi), and Redi Cube (Moyu, or Yuxin 8 Petals Cube) are all pretty intuitive puzzles that aren't too hard to solve. FTO (LanLan) is pretty popular and unique, although the hardware is terrible. Curvy Copter (also LanLan) is extremely unique as it is ajumbling puzzle, though it takes a while to solve and the hardware is terrible. Mirror Cube (the Gan fits in your $30 budget, but if you don't want to spend that much get the MFJS) and Fisher Cube (Qiyi or YJ) are both great introductions to the world of shapemods. Kilominx (Qiyi, but get the stickered one) is just a smaller Megaminx, and Master Pyraminx (Yuxin) is self-explanatory. Gigaminx would be a good addition but it's outside your price range; same with the Mixup Cube (which is out of stock anyway). Gear Cube (Qiyi) looks intimidating but is actually really easy; I solved it accidentally multiple times. 15 Puzzle (Qiyi) is worth a go if you'd like something non-twisty. I'd recommend the Rua (LanLan), but it isn't on TC.