You don't need all the equipment to get a full body exercise. If you've got just a set of dumbbells and looking to not lose your gains, you just need to change your Speed, Rep Scheme, and Time Duration!
Change Speed - If you've got only one set of dumbbells and certain movements are easy because it is too light (probably a row or squat of some kind), then there are ways you can change the speed to make it more difficult
Slow Down - Add a tempo to the movement. If you're row a 20# dumbbell, take 10 seconds to bring it to your chest instead of the usual .5 second. 6-10 of these tempo rows will feel like you got a serious arm day in.
Speed Up - Add speed to a movement to make it more challenging. If you're squatting a 20# dumbbell, take 10 seconds to go down, pause at the bottom, and drive up as fast as you can.
Stop (Isometric Hold) - Simply holding your muscles in contraction for a long period of time is extremely difficult. If you're rowing your 20# dumbbell, hold it at the top position for 30 seconds, relax for 10 seconds, then repeat until failure.
Mix in a combination of all of these! Squat down slow for 10 seconds, hold the bottom position under tension for 5 seconds, then drive up as fast (or as slow) as you can!
Change Rep Scheme - Not only can you change the number of repetitions, but you can change the number of sets you do as well.
Ascending Set - Start each set at 2 and go up by 2 each new round; or start at 5 and work your way up by 5s; or go up by the number squared (though that one will get tough REAL quick, ie, 3, 9, 27, 81, 243, yikes).
Descending Set - Start each set at certain number and go down by a certain number. I'm a big fan of 21-15-9. Starts hard and finishes with a smaller number (not necessarily easier though). You can do 10-8-6...
Pyramid Set - Start low and work your way high and since you had so much fun the first time through those numbers, might as well go back from where you came from! 3-6-9-12-15-18-21-18-15-12-9-6-3. You can start low or high with these and work the opposite direction! That'll get saucy!
Plateau Set - Perform the lift at a low number then a high number for a few sets, then a low number. For instance, 3-8-8-8-3-8-8-8-3.
Failure - Do multiple sets to failure. These are good for body weight things that are easy to start, but then get real hard after a few sets. I could easily knock out three sets of 20 push ups, but ask me to 3 sets of failure push ups and my numbers would likely be something along the lines of 30, 18, 11.
Change Time Duration - Not all working sets have to be done as set number of reps. Here are some thoughts on how to use a clock to spice up your workout.
Tabata - A traditional tabata is :20 of work and :10 of rest for 8 rounds at one exercise, then do 3 other exercises for a total body slam sesh in less than 20 minutes. Push Ups, Sit Ups, Pull Ups (if possible), Body Weight Squats. The idea is to get as many reps as possible in that time period.
X Amount of Time On, X Amount of Time Off - I like setting up a short cycle of exercises (such as 5 push ups, 8 sit ups, 11 body weight squats) and seeing how many times I can get through it in 2 minutes, with 90 seconds of rest; I'll then repeat for 4 more rounds.
Death by (Insert Exercise) - This is an ascending rep scheme within a time domain. One rep the first minute, two the second, and so on... until you can't complete the number of reps in that minute. Your reps go up and rest goes down. Usually by the time you get to 12 or 13, you're gassed. A classic one I really like is Death by DB Thrusters.