Assorted Tips for keeping the summer learning alive!

Reading:

My son and I visit the library each week, and as we read each new selection of books, we keep his word study practice going of seeking out words with particular characteristics (for example, sh, ch, th).  In this way he retains the habit of 'doing homework' each day, and also expands his ever-growing vocabulary.
We also select a combination of books that he reads by himself, and ones that are read to him.  Generally we read three to four books each evening at bedtime.

He is also subscribed to a kid's magazine, and he reads that on his own over the course of a month.  These magazines are also useful to take along on longer road trips, to keep him entertained.

Spanish:

My son is in a dual language school environment (English/Spanish), and it is thus imperative that we practice his second language (Spanish) consistently during the summer.  We focus on one room per week, e.g. the kitchen, then the bathroom, to learn and use the Spanish words related to that room.  In that way, repetition enforces remembering the vocabulary.  We also use familiar phrases constantly, e.g. Wash your hands, It's bedtime!

Geography:

As we plan our upcoming vacation to various states, we involve our son in planning the trip.  This involves learning where the various states are located, how many miles it takes to travel there, and (incorporating some math), how long he'll most likely be sitting in the car!  We also review telling time, thinking about what time we will leave, how long the trip will  be, and thus what time we're likely to arrive at our destination.

Overall Responsibility/Completing Tasks on Time:
Our son just turned six, and was asking for a mini-allowance.  We decided on two tasks that he will be responsible for on a daily basis: taking products appropriate for recycling down to the appropriate bins, and setting the table for each dinner meal.  Upon satisfactory completion of these tasks, he receives 50c (2 quarters) at the end of each week.  He is learning to monitor his own behavior, to ensure he does the tasks without constantly being asked.  He is also aware that he has to continue to do other tasks (e.g. place his clothes in the hamper each evening) as a  member of the household, and these are not 'rewarded' monetarily.  Bonus: as his teeth are falling out, the tooth  fairy adds in some additional income, which provides another opportunity to practice his addition!

Math:

As we've just begun providing our son with a mini-allowance for completing his assigned tasks each week, we use this activity to encourage him to add up how much he is getting each week (2 quarters).  He also assists in the kitchen with cooking and baking, and during those activities he learns about measurements as he assembles the ingredients for pancakes, or measures out the appropriate ingredients for cupcakes for a visiting grandmother's birthday!  Real world applications are vital to keeping him engaged and eager to indulge in math, and these kinds of activities are fun for  him.   Also, as he plays with building blocks, constructing a tall tower, we count the blocks, in English and Spanish, to keep his counting skills alive.  We also work on subtraction, removing blocks and ascertaining how  many are left.