While probably the most obvious, the run-on-the-beach option endures as an excellent way to add some natural beauty and resistance into your regular running routine. If you aren't into sandy toes, you'll want to have some beach-dedicated shoes and find the hard-packed, wetter sand. While not as challenging as a barefoot, soft-sand approach, the absorption of wet sand is still significantly different than traditional paved surfaces, so you should notice more effort is required to propel you forward.
If you don't mind the sandy toes, leave your shoes behind and alternate between the hard pack and soft, either for time (30 to 60 seconds on each) or by distance (between lifeguard stands or other regular markers). Your feet will sink into the soft sand much more, and the movement will require more from your ankles as far as mobility and balance, so only do this option if your ankles are up for it. Your toes will have to grip more, and the uneven surface will require more activation from your lower legs, especially your calf muscles. Running on soft sand will be more tiring, which is why a hybrid hard-sand/soft-sand technique may help you cover more distance and feel more successful.
Not a runner? These same principles apply to walking — just remember that you are working out, not strolling. Walk at a pace where you feel your heart rate quicken, your rate of breath pick up, and some activation in your lower-body muscles. Add in a vigorous arm swing and you'll take it from a walk to a full-body workout.