Three possible topics:
(1) Tides.
Not easy to understand (it awaited Newton), and
somewhat intricate. But you could present some
mathematical justification, using, e.g., the discussion
here.
Once you understand tides, you should be able to convince them
that their own weight varies in concert with the moon's tidal pull,
and maybe calculate that variation (~$10$ mg),
from
$$F = G \, \frac{m_1 m_2}{r^2}$$
as a project.
Make that a goal: Understand tides well enough to realize our
weight changes in concert with the tides; and compute that (miniscule) change.
(Image from a Jim Brau page.)
(2) Evaporation.
The evaporation rate (at fixed temperature, fixed pressure) is
proportional to surface area. At room temperature, with no wind,
normal humidity,
water evaporates at (very!) roughly
$4 \times 10^{-5} \, \mathrm{kg/s/m}^2$
(kilograms of water per second per meter squared of surface area).
From this one can calculate how long it would take a glass of water to
evaporate, or a swimming pool, or a lake:
(Image from KidZone.)
Those calculations could form a project: Understand the consequences
of evaporation time-linear w.r.t. surface-area, and compute several
common instances.
(Spherical raindrop evaporation would be quite challenging for 7
th-graders.)
(3) Waves.
Generally 7th-graders (in the U.S.) have no exposure to trigonometry,
but fortunately ocean waves are not sine waves, but instead
better approximated by
trochoids,
for physical reasons
(which one can sense while ocean-bathing):
(Image from HyperPhysics.)
I can imagine a project that explores trochoids
both geometrically and through computational simulations.
There could also be a nice experimental component:
measure the speed of water waves (
celerity),
and predict how long it would take an ocean wave to
travel from coast
$A$ to coast
$B$—from Cairo to Cyprus,
or whichever geographical locations are most salient to the students.