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Top picks: Joybble 20oz Stainless Steel Travel Tumbler Gift Set (best overall); Festivora 21st Birthday Gamer Gift Set if he spends his evenings gaming; YBZZRAG Mens Leather Bracelet if he likes understated accessories.
By 21 he may be at university, in a first job or juggling both, so think about where the present will end up: his desk, his keys or his room. Under £30, aim at one real interest of his, such as gaming.
The Joybble set gives him an insulated travel tumbler, an engraved metal keyring and a multitool pen, all things that fit a busy working or student week. It comes in a gift box.
Worth knowing: Skip if he already carries a reusable cup everywhere.
The Festivora box is built around gaming, with a "LEVEL 21 UNLOCKED" tumbler and a pillowcase reading "GAMER'S HAPPY PLACE" on one side and "BIRTHDAY BOY" on the other.
Worth knowing: Skip if he does not play video games.
The YBZZRAG bracelet pairs black braided leather with stainless steel, a plain style he can wear casually or with smarter clothes.
Worth knowing: Skip if he never wears anything on his wrist.
The musaecoin crochet toast makes a pun of raising a toast to his future, a light-hearted handmade keepsake for a desk or shelf.
Worth knowing: Skip if he would find it too cute.
The NICECONE socks carry the line "It Took 21 Years to Look This Good", a small joke to hand over beside a bigger present.
Worth knowing: Skip if he only wears plain socks.