Hi..

I have a spare 29g, and I thought it would be cool to
create a vivarium. I would like to have land with the
appropriate plants, and a "swimming" area for a couple of
small fish.

I suppose you mean a palludarium.

The following Austrian link (in German) shows a pretty
_open_ palludarium..:

http://members.chello.at/gerhard.klotz/page16.html

An easy to manage _closed_ palludarium for example is an
ordinary aquarium with a lamp hood..

I had a closed 110l palludarium, too..:

# 39cm / 15" hight
# 2" black basalt gravel (2-5 mm / 0.1-0.2")
# 5" water zone
# finally 8" land zone
# water zone heavily planted.
# land zone with a big branched book root and several
aquaria plant species what are able to grow outside the
water
# land zone plants grew in a long and tall plastic flower
box for the window sill with a dark brown oak surface
filled with natural 0.1" quartz gravel.
# flower box stood onto the basalt gravel bed in the
background.
# inner side of the rear pane was completely sticked with
peat plates..
# sucked peat plates look very natural and were an ideal
rooting ground for emerse growing ("kreeping") aquaria
plants and mosses..
# used a tiny water pump that pumped the tank water into the
emerse planted flower box..

I would like to keep a couple of frogs,

Dwarf African Frogs (Hymenochiris )..?
Why not..

and have plants that thrive in a moist environment.

A lot of well known aquaria plants are bog plants and are
able to grow outside the water, too..

Is a 29 to big?

Well.., tank-related there is _no_ "too big"..! :-)

This could be a fun project

Have fun..!

P.S.: Don't know where you are from but evaporation, air
moisture *and* lower room temperature might cause dewy
glass panes. Personally I had a 200 l/h air pump. The air
hose ended in the _land_ zone and changed the air
continuously. Alternatively a PC ventilator with a time
switch might be fine, too. As far as I remember others used
glass pane heaters..

HTH.
--
cu
Marco