Trying to think of an appropriate thank you gift...

[Firegriff]

So recently, a friend of mine who played Magic a long time back gave me his collection of cards (most recent set thereinbeing Mirage).  Included in the cards were a full set of Megrim, Ensnaring Bridge, Mana Web (insanely broken card, but legal in EDH) and Grave Pact.  Just trying to think of a good way to show my appreciation.

[Mishra]

Well....we don't know in what kind of hobbies your friend is into. Since he gave his collection to you I am assuming he is not playing anymore. So anything Magic related might not be that appropriate.

However that reminds me of a gift my brother in law got from me christmas 2003.
Bought him a playset of DCI foiled Judge Reward Gaea's Cradle and 4 acrylic card containers that can be sealed by screws. Drilled holes in the acrylic parts (of course not into the cards, LOL) and linked them with thin chains. That way he was able to hang em on the wall for decorative purposes. If I'd only had known what price explosion those would undergo. (They were like 40 bucks a piece back then)

[CardAgain Sweater]

Agree with poster above, not knowing your friend's interests means any suggestion will be less personal; having said that, although the friend quit magic (it sounds like) they did at one time play Nerd Cards. This leads me to believe they may like other Nerd Things. Get them a piece of a meteorite - they are crazy affordable considering what they are, and there are verifiable source out there you can trust. The iron-nickel ones are my favorite, as they actually look like space stuff; the condrites are cool too, but kinda just look like rocks unless they are split or heavily fused from entry.

I got one for myself and love the fact that I own a piece of something from space that zipped around up there for a few billion years or more, just happened to cross paths with _only_ our planet, at a time in evolution and in a specific locality to allow humans to observe it, that there was someone who did in fact observe the fall or find its aftermath, and now I carry the result of this string of improbabilities in my pocket with my spare change.

It also makes a handy impromptu paperweight for mild to moderate breezes.

[Firegriff]

The cards also had 3 Culling the weak.  One went into a Thraximundar EDH with Attrition and Gravepact.  Another went into a Nath of the Gilt-Leaf EDH deck with Mind Peel.  Eee gad, it's a mean combo!  It was no trouble at all forcing the two opposing decks out of cards! . A counter spell would be about the only way to not keep the cycle going every single turn...