Guus
The best site for plants is the isle of Comino and Ocober will be perfect for bulbs I've been at that time and saw a great deal. Easily reached by ferry and/or motor launch. Also cliff tops at Gozo.
For wildlife try Il-Buskett woodlands, take bus #81 from Valleta. I think it's the only substantial wood on the island and usually has a fair selection of birds. It is a small area by northern European standards (and popular).
There is also Is-samir wetland reserve near Bugibba but you need to contact Birdife Malta to get access to the hides though you can still see things from the perimeter (contact details at
www.birdlifemalta.org). Good for bitterns
Gardens at Palazzo Parisio
www.palazzoparisio.com are small but perfectly kept and the palace is a joy to behold. There is also the botanic garden at San Anton Palace, Attard (bus #40 from Valleta) - not terribly exciting though it has a rococo fountain and some interesting tree specimens as well as usual tropical exotica and worth a short detour.
Balconies and steps are packed with pots and are a god starting point for looking for succulents with many escapees now naturalised - there is often a good cacti or succulent show happening somewhere on the island each week.
Frazer