As beer continues to take over DC, so do tour options that don't involve riding around in a bus, getting yelled at by a failed comic with a megaphone. DC Brew Tours offers tour packages that'll whisk you safely from brewery to brewery, giving you the chance to sample from the best of the city's excellent local brewing scene, including DC Brau, Capitol City Brewing, and Right Proper. Both the daytime and evening rides include full beer-pairing meals in addition to the VIP tours, which kind of makes these rides a totally good value at $90 a pop.
If your cousins really need to see all the monuments, a kayak tour has to be the least boring way for you to experience them. The tour includes instructors who will teach you what you need to know about paddling, and guides who will give you the full rundown on all the monuments you pass (and other items of note). Uh, make sure everyone in the party can swim, first. Reserve a spot on the website and be prepared to pay $95 apiece.