Assuming you want them all to come on together, with a higher than 5V supply, you can use a series-parallel combination.
For example, let's say you have a 12V power supply and your LEDs are rated at 3.5V. If you put two in series, that's 7V across the LEDs and 5V across the resistors. With 30 LEDs, you'd wire-up 15 sets of those. (The more you wire in series, the more efficient your system with less power wasted in the resistors.)
With a resistor current limiter, it's generally best (best for current/brightness control, but not for efficiency) if you drop about half the voltage (or more) across the resistor. So if you want to go with 3 in series (10 sets), I'd say use at least a 15V power supply.
- How to power the LEDs? (the battery must not be too large because it needs to fit on a garden sculpture)
You might want to look-up some Amp-hour ratingsfor various batteries and do some Amp-hour calculations, based on the current required for the LEDs plus the Arduino and other circuitry.
If you're not up-to-speed on [u]Ohm's Law[/u] and [u]Kirckhoff's Laws[/u]... Briefly, the same current flows through series components. So, two LEDs in series share the same current, and use half the current of two LEDs in parallel (at the same brightness).
And, a constant-current switching LED power supply doesn't waste power heating-up current-limiting resistors. That means your battery would last longer (at the cost of a more-expensive, or more complex, power supply).