generally what you want for rap is some kind of percussion between some of the beats. in that sense you can use any chords (of the same key) an example being that you can switch between D and G with tapping the body of your uke between.
other percussive effects you may want to consider using are stomping your feet, playing a muted chord, or lightly slapping your uke with an open palm. (and you can always have someone else do the percussion through beatbox or whatever else is available)
You should also consider doing bass lines if you have the coordination. One that is has been used with great success is the bass line from in-a-gada-da-vida
I play it as this
PHP:
A----------------------------------------------------
E---------1--0---------------1--0--------------------
C---2--2--------0---2--2--2--------0-----------------
g----------------------------------------------------
A----------------------------------------------------
E---------1--0---------5--4--3-----------------------
C---2--2--------0---2--------------------------------
g----------------------------------------------------
example of its use being
http://www.youtube.com/watch?v=e9vYDJCm63c
They only use the back half of the bass line, i think it may be more effective that way.
Of course you can certainly use a lot of unexpected sources for a good back ground beat. I've had people rap while I played misirlou (the black eyed peas did it here to,
http://www.youtube.com/watch?v=TwZ6Jv2v_D4&feature=PlayList&p=475271F756E67CED&index=0&playnext=1) there's a tab for it here
http://www.muffin.net.nz/muffin/Pdf_Tablature_files/misirlou.pdf
The tabs from there site you generally would want to supplement with play the melody on top of the chords in some places to give it more of a body.
A side note: there's plenty of chords for raps songs out there, a google search will give you the chords to specific ones, if you start off a rap that people recognize they will probably have an easier time picking it up after you finish