You can try:
1. Buddhist Temple at Race Course Road. This is located nearer Balestier Road end.
2. Temple at Kim Yian Road, off River Valley Road. Road directly opposite the Automobile Association (AA) Building. Just about 100m from the junction.
3. Thai temple along Still Road, road parallel to Changi Road just before Geyland Road. Access by MRT is possible, get off at Eunos Stn. (may be wrong, but it's around there). You should be able to see the temple from the Stn. Every year on Vesak Day, lots of activities there.
"Four Horse Road" will be very crowded. Activities there would be because it's the 15th. day of the Chinese calendar, not because of Vesak Day.