Souvenirs are tricky things. You want something to commemorate your journey, but you don't want to end up with a bunch of useless junk, or with things so cumbersome that they won't go in your bags. Here are some of my best ideas of what to purchase.
Postcards: not only can they be used in scrapbooks, but they can also be framed or you can get really crafty and use self-adhesive magnetic sheets and turn them into magnets. This generally works best with the shaped postcards.
Tee-shirts: not only are they a fun souvenirs but they will also work as cushions for more fragile items. You can always buy small or extra small shirts and turn them into pillows, either tee-shirt shaped or any other shape by cutting up the shirt.
Jewelry: small and generally light this souvenior will travel well. It can also break the budget if you aren't careful. Be choosy and only buy stuff you, or the gift recipient, will actually wear.
Magnets: My sister and I both collect these and it is always fun to add to a collection. These are easy to find and not outrageously priced. They will also serve a function which adds to their appeal.
Tea Towels: You're probably thinking, what the? But once again they are useful items that will travel well. Aprons are another fun idea, but only if you are a cook or know someone who is.
Edibles: Avoid anything that is packaged in glass because of the added weight and the likelihood of breakage. These are quite fun to bring back but they are consumable and thus shouldn't be the only thing you bring home with you. Plus, the more you bring the greater your chances of having to deal with duties.
Bet you thought I would suggest books huh? As tempting as they can be, (I'm still kicking myself for not buying the English to Pirate phrase book in Bath, England last summer), they are bulky and weigh a lot. If it isn't something you can typically get in your home country, without paying outrageous shipping, go ahead and buy it. But be forewarned, you may have to pay extra to get your checked bag home.
If you are driving to your destination then the types of souvenirs you can safely bring home increases exponentially. Just make sure to leave enough room for the passengers.