I think getting them a BT21 item will be the best gift (BT21 are the characters created by BTS in their collab with Line). So I advise you to know their favorite character and get them one of these:  

• BT21 Christmas Limited Winter Collection Plush Doll HERE
• BT21 official wireless mouse (This is totally cute and useful) HERE
• BT21 official wireless keyboard HERE
• BT21 Universtar Diary (You can get them this if they need one) HERE
• BT21 Official flat cushion HERE
• BT21 X VT Lippie stick (If they are into makeup) HERE
• [LIMITED EDITION PUMA X BTS] BTS Basket Patent Sneakers (This will make them fly beyond the clouds with happiness) HERE
• And so much more official goods HERE

You can use in all of these, special coupon codes: mimibtsghost1 for 10% off on your whole order or mimibtsghost2 for free worldwide shipping on your whole order ^^