I have no idea of when your 5 free hours fall, but there's a shuttle from the Galleria mall to Kendall Square (stops in front of the Marriott). This is also walking distance (10-15 minutes?) if you walk like a non-American, but if you're the car type, it may be a longer stroll than you are up for.

In Kendall Square, you can grab a snack or meal at Legal Seafoods, wander the tech areas, or visit that corner of MIT's campus. Actually, the brand new Stata Center designed by Frank Gehry is just a few blocks down Broadway (away from the river, towards the center of Cambridge)....It's the bizarre looking building that seems to have melted or been through an earthquake The center of MIT's campus is behind those Broadway buildings and also makes an interesting walk.

Of course, if you really want to see Cambridge, you should visit MIT, then walk or T (local name for the subway) from Kendall Sq to Central Sq, wander down to Harvard Square, and then take the T back. Kendall, Central, and Harvard Sqs are all on the red line, while the Royal Sonesta is on the green line with a not-terribly-convenient connection (requiring going into Boston).

Of course, the science museum or strolling along the river are great suggestions too!