I am a beginner ukulele/guitar player and although I'm not a great musician I enjoy making up my own songs. I've started improvising a bouncy, twee little tune with a C-F-G7-C progression but not sure what other chords I can add in to spice it up. Hopefully easy ones... Any suggestions? Thanks!
The world is your oyster. You could add any chords you like, but some choices are much more likely to give an effect you will like the sound of!
As @Dom says in his comment, other chords from C Major will work well. This is because C, F and G are the primary triads of C Major. The other triads are: Dm Em and Am. (There is also Bdim, a diminished triad, but you might want to leave this for now. Besides, it is contained within your G7 chord anyway.)
Beyond these triads there are a number of ways to find other chords that might work for you:
you could extend these C Major triads by adding, for instance, sevenths. This would give: Cmaj7 Dm7 Em7 Fmaj7 G7 Am7
you could "decorate" your basic chords by adding 9ths or 6ths to them, or by alternating them with suspended chords (e.g. sus2 or sus4 chords). An example would be playing C Csus4 C Csus2 C C6 instead of six C chords
you could "borrow" some chords from keys closely related to C Major, or even use these chords to modulate to a new key. For instance, for a darker turn to your chord sequence, you could head towards A Minor, the relative minor of C Major, by using chord V of A Minor, which is E. Alternatively, borrowing a D Major chord from G Major, the dominant of C Major, might work well. This is called a secondary dominant. (Chord V is known as the dominant, so D is the dominant of the dominant, G.) Bm could be "borrowed" from G Major, too. You could borrow chords from F Major, the sub-dominant key of C Major. This would usefully give you Bb and Gm chords.
These are just a few obvious ideas, but hopefully should suggest some avenues for experimentation. Don't rule out less obvious choices: there are plenty of great chord sequences that don't follow the kind of "rules" outlined above. A personal favourite of mine is a sequence of major chords, whose root notes are related by the notes of a minor scale. Smells Like Teen Spirit by Nirvana is an obvious example. Its introduction uses these chords: F Ab Bb Db (although not in that order...) Taking C as your home key, this would suggest using: C Eb F Ab, and maybe Bb, D or G too...
The possibilities are endless!
Using C, F and G7 gives you the main chords that will be found in almost all tunes played in C major. There are three minors which also belong to that key, Am, Dm, and Em.(Actually the three relatives of the original chords). They will all be good to use. The remaining chord straight from C is B diminished, which isn't quite as usable as the other 6, but is made up from B, D and F.You could also use D7, especially if G followed, or E7, especially if Am followed.